Coding pattern comprising registration codeword having variants corresponding to possible registrations

ABSTRACT

A substrate having a coding pattern disposed thereon or therein. The coding pattern comprises a tiling of contiguous grid cells, each grid cell being demarcated by t target elements and having t-fold rotational symmetry, each grid cell containing nt registration symbols, each registration symbol being encoded by a set of macrodots; and a tiling of contiguous tags, each tag consisting of an array of c grid cells, each tag containing a plurality of data symbols. There are ct possible registrations between a physical layout of the coding pattern and a logical layout of the coding pattern and any contiguous tag-shaped array of c grid cells contains cnt registration symbols. The registration symbols, taken in a defined sequence relative to the physical layout of the tag-shaped array, form a registration codeword of length r. There are v distinct registration codewords, each corresponding to a distinct one of the ct possible registrations.

FIELD OF INVENTION

The present invention relates to a position-coding pattern on a surface.

COPENDING APPLICATIONS

The following applications have been filed by the Applicantsimultaneously with the present application:

NPT118US NPT119US NPT120US NPT121US NPT122US NPT124US NPT125US NPT126USNPT127US NPT128US NPT129US

The disclosures of these co-pending applications are incorporated hereinby reference. The above applications have been identified by theirfiling docket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

10/815,621 10/815,635 10/815,647 11/488,162 10/815,636 11/041,65211/041,609 11/041,556 10/815,609 7,204,941 7,278,727 10/913,3807,122,076 7,156,289 09/575,197 6,720,985 7,295,839 09/722,174 7,068,3827,094,910 7,062,651 6,644,642 6,549,935 6,987,573 6,727,996 6,760,1197,064,851 6,290,349 6,428,155 6,785,016 6,831,682 6,741,871 6,965,43910/932,044 6,870,966 6,474,888 6,724,374 6,788,982 7,263,270 6,788,2936,737,591 09/693,514 10/778,056 10/778,061 11/193,482 7,055,7396,830,196 7,182,247 7,082,562 10/409,864 7,108,192 10/492,169 10/492,15210/492,168 10/492,161 7,308,148 6,957,768 7,170,499 11/856,06111/672,522 11/672,950 11/754,310 12/015,507 7,148,345 12/025,74612/025,762 12/025,765 10/407,212 6,902,255 6,755,509 12/178,61112/178,619

BACKGROUND

The Applicant has previously described a method of enabling users toaccess information from a computer system via a printed substrate e.g.paper. The substrate has a coding pattern printed thereon, which is readby an optical sensing device when the user interacts with the substrateusing the sensing device. A computer receives interaction data from thesensing device and uses this data to determine what action is beingrequested by the user. For example, a user may make make handwritteninput onto a form or make a selection gesture around a printed item.This input is interpreted by the computer system with reference to apage description corresponding to the printed substrate.

It would desirable to improve the coding pattern on the substrate so asto maximize the data capacity of the coding pattern and minimize theoverall visibility of the coding pattern on the substrate. The codingpattern typically comprises data symbols (encoding ‘useful’ information,such as an identity and/or a location) and as well as other symbols,which allow the data symbols to be decoded. It would be desirable tominimize the amount of data encoded in these other symbols so as to makemaximum use of the data symbols. By minimizing the space occupied bythese other symbols, there is more space available in the coding patternfor data symbols which encode useful information.

SUMMARY OF INVENTION

In a first aspect, there is provided a substrate having a coding patterndisposed thereon or therein, the coding pattern comprising:

-   -   a tiling of contiguous grid cells, each grid cell being        demarcated by t target elements and having t-fold rotational        symmetry, each grid cell containing nt registration symbols,        each registration symbol being encoded by a set of macrodots;        and    -   a tiling of contiguous tags, each tag consisting of an array of        c grid cells, each tag containing a plurality of data symbols        and having an identical layout of data symbols, each data symbol        being encoded by a set of macrodots;        wherein:

the coding pattern has a physical layout defined by its tiling ofcontiguous grid cells, the physical layout belonging to a plane symmetrygroup that has t-fold rotational symmetry and translational symmetrywith the grid cell as its unit cell;

the coding pattern has a logical layout defined by its tiling ofcontiguous tags, the logical layout belonging to a plane symmetry groupthat has no rotational symmetry but has translational symmetry with thetag as its unit cell;

there are ct possible registrations between the physical layout of thecoding pattern and the logical layout of the coding pattern, eachregistration corresponding to a distinct combination of one of the tpossible rotations of the physical layout of coding pattern relative tothe logical layout of the coding pattern and one of the c possibletranslations of the physical layout of the coding pattern relative tothe logical layout of the coding pattern;

any contiguous tag-shaped array of c grid cells contains cntregistration symbols, the registration symbols, taken in a definedsequence relative to the physical layout of the tag-shaped array,forming a registration codeword of length r;

there are v distinct registration codewords, each corresponding to adistinct one of the ct possible registrations;

the registration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between the tag-shapedarray and the logical layout of the coding pattern;

t=is an integer value of 2 or more;

c=is an integer value of 2 or more;

n is an integer value of 1 or more;

cnt≧r; and

v≧ct.

The coding pattern according to the first aspect advantageouslyminimizes the data capacity requirements of the registration symbols.This is achieved at least in part by the symmetry of the coding patternand, in particular, the symmetry relationship between ‘grid cells’ onthe one hand, and ‘tags’ on the other. Moreover, each tag-shaped arrayof grid cells encodes a registration codeword variant, with each variantidentifying a distinct registration between the tag-shaped array and thelogical layout of the coding pattern (having a tag as its unit cell).The particular registration codeword variant determined when decodingthe coding pattern enables the registration to be accurately determined

Optionally, n is 1, 2, 3 or 4. Optionally, n=1. In other words, eachgrid cell typically contains t registration symbols.

Optionally, r=ct. In other words, the number of possible registrationstypically matches the length of the each registration codeword.

Optionally, t is an integer from 2 to 36. Optionally, t is 2, 3, 4 or 6,corresponding to linear, triangular, square and hexagonal grid cells.

Optionally, c is from 2 to 64. Optionally, c is 2, 3, 4, 9, 16, 25 or36, corresponding to 2, 3, 2×2, 3×3, 4×4, 5×5 or 6×6 grid cells per tag.

Optionally, the registration codeword of each tag-shaped arrayfacilitates identification of at least one of the data symbols of a tagthat shares at least one grid cell with the tag-shaped array.

Optionally, v=ct. In other words, the number of registration codewordsmay match the number of possible registrations.

Optionally, the defined sequence is different for each of the ctpossible registration codewords. Optionally, a minimum distance betweeneach of the ct distinct registration codewords is at least r/2, therebyallowing the registration between the tag-shaped array and the logicallayout of the coding pattern to be determined in the presence of up to(r/2−1)/2 registration symbol errors.

Optionally, v>ct, such that each of the ct possible registrations has aplurality of corresponding registration codewords.

Optionally, a minimum distance between pairs of distinct registrationcodewords encoding different registrations is at least r/2. This ensuresrobust determination of registration, even when there are two or morepossible formats.

Optionally, each of the ct possible registrations has first and secondcorresponding registration codewords, the first registration codewordsidentifying a first encoding format of the coding pattern and the secondregistration codewords identifying a second encoding format of thecoding pattern.

Optionally, a minimum distance (d_(format)) between each pair of firstand second registration codewords encoding the same registration is atleast r/2. This ensures robust determination of the format.

Optionally, each data symbol is encoded using multi-pulse positionmodulation.

Optionally, each data symbol is represented by d₁ macrodots, each of thed₁ macrodots occupying a respective position from a plurality ofpredetermined possible positions p₁, the respective positions of the d₁macrodots representing one of a plurality of possible data values, andwherein d₁ has different values in the first and second encodingformats. Optionally, p₁ is from 4 to 12 and d₁ is from 2 to 6.Typically, p₁ is 7 and d₁ is either 2 or 3, corresponding to a firstformat with 2-7PPM encoding of data symbols and a second format with 3-7PPM encoding of data symbols.

Optionally, each of the ct possible registrations has a correspondingflagged first registration codeword, a corresponding unflagged firstregistration codeword, a corresponding flagged second registrationcodeword and a corresponding unflagged second registration codeword, theflagged registration codewords identifying the presence of an activearea flag and the unflagged registration codewords identifying theabsence of the active area flag.

Optionally, at least some tag-shaped arrays in the coding pattern havingthe first encoding format contain a mixture of registration symbols fromthe flagged first registration codeword and registration symbols fromthe unflagged first registration codeword to form a mixed firstregistration codeword of length r; and

at least some tag-shaped arrays in the coding pattern having the secondformat contain a mixture of registration symbols from the flagged secondregistration codeword and registration symbols from the unflagged secondregistration codeword to form a mixed second registration codeword oflength r.

It will be appreciated that a mixed first registration codeword or amixed second registration codeword is ambiguous in respect of thepresence or absence of an active area flag. Typically, the active areais assumed to be present during decoding in the event of such ambiguity.

Optionally, a minimum distance (d_(flag)) between each pair of flaggedand unflagged registration codewords encoding the same registration andthe same format is less than r/2, thereby enabling correction ofregistration symbol errors in the mixed first and second registrationcodewords. Hence, registration and format encoding is prioritized overflag encoding.

Optionally, each tag contains a registration codeword selected from oneof the codewords defined as:

tag active area codeword codeword type format flag 4414, 0332, 2253,1132, 1411, first; unflagged 0 0 3402, 5503, 2113, 2155 2414, 3332,4253, 4132, 4411, first; flagged 1 5402, 4503, 1113, 3155 2321, 5555,1101, 1212, 5500, second; unflagged 1 0 4540, 5531, 4311, 4512 0321,3555, 0101, 0212, 4500, second; flagged 1 5540, 1531, 5311, 5512

These codewords are optimized so as to provide a maximum distancebetween codewords encoding different formats, and a relatively smallerdistance between flagged and unflagged codewords encoding the sameformat.

Optionally, each registration symbol by a multi-pulse positionmodulation (PPM) encoding. Optionally, each registration symbol isrepresented by d₂ macrodots, each of the d₂ macrodots occupying arespective position from a plurality of predetermined possible positionsp₂, the respective positions of the d₂ macrodots representing one of aplurality of possible registration symbol values. Optionally, p₂ is aninteger from 4 to 12 and d₂ is an integer from 2 to 6. Optionally, p₂ is4 and d₂ is 2, providing six different registration symbol values perregistration symbol.

Optionally, each tag comprises at least one local codeword identifying alocation of a respective tag, the local codeword comprising a respectiveset of the data symbols.

Optionally, each tag comprises one or more common codewords, each commoncodeword being common to a plurality of contiguous tags, each commoncodeword comprising a respective set of the data symbols.

Optionally, each common codeword, or a set of common codewords,identifies an identity. The identity typically identifies at least oneof: the substrate; a region; a page; a product; a visual layout; and aninteractivity layout.

In a second aspect, there is provided substrate having a coding patterndisposed on or in a surface thereof, the coding pattern comprising:

-   -   a tiling of contiguous grid cells, each grid cell being        demarcated by t target elements and having t-fold rotational        symmetry, each grid cell containing nt registration symbols,        each registration symbol being encoded by a set of macrodots;        and    -   a tiling of contiguous tags, each tag consisting of an array of        c grid cells, each tag containing a plurality of data symbols        and having an identical layout of data symbols, each data symbol        being encoded by a set of macrodots;        wherein:

the coding pattern has a physical layout defined by its tiling ofcontiguous grid cells, the physical layout belonging to a plane symmetrygroup that has at least one reflection axis, t-fold rotational symmetryand translational symmetry with the grid cell as its unit cell;

the coding pattern has a logical layout defined by its tiling ofcontiguous tags, the logical layout belonging to a plane symmetry groupthat has no reflection axis and no rotational symmetry, but hastranslational symmetry with the tag as its unit cell;

there are 2ct possible registrations between the physical layout of thecoding pattern and the logical layout of the coding pattern, eachregistration corresponding to a distinct combination of: (1) whether ornot the physical layout of the coding pattern is reflected relative tothe logical layout of the coding pattern; (2) one of t possiblerotations of the physical layout of coding pattern relative to thelogical layout of the coding pattern; and (3) one of c possibletranslations of the physical layout of the coding pattern relative tothe logical layout of the coding pattern;

any contiguous tag-shaped array of c grid cells contains cntregistration symbols, the registration symbols, taken in a definedsequence relative to the physical layout of the tag-shaped array,forming a registration codeword of length r;

there are v distinct registration codewords, each corresponding to adistinct one of the 2ct possible registrations;

the registration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between the tag-shapedarray and the logical layout of the coding pattern;

t=is an integer value of 2 or more;

c=is an integer value of 2 or more;

n is an integer value of 1 or more;

cnt≧r; and

v≧2ct.

Coding patterns as defined in connection with the second aspect areuseful for transparent substrates (or at least partially transparentsubstrates) wherein the coding pattern may be read from either side ofthe substrate. In this case, the number of possible registrationsdoubles, because the coding pattern may be read either unreflected (i.e.from a first side of the substrate) or reflected (i.e. from a secondside of the substrate). The length r of the registration codeword may bethe same as that in the first aspect; however, the number of distinctregistration codewords (or variants) is typically doubled to account forthe additional reflection within the registration. In other words,optionally r=2ct. Although the number of distinct registration codewordsmay double for the same length of codeword, the code is optimized sothat distinct registration codewords are maximally separated and robustdetermination of registration is still possible.

Other optional embodiment in connection with the second aspect mirrorthose optional embodiments in connection with the first aspect.

Optionally, v=2ct.

Optionally, a minimum distance between each of the 2ct distinctregistration codewords is at least r/2, thereby allowing theregistration to be determined in the presence of up to (r/2−1)/2registration symbol errors.

Optionally, v>2ct, such that each of the 2ct possible registrations hasa plurality of corresponding registration codewords.

Optionally, a minimum distance between pairs of distinct registrationcodewords encoding different registrations is at least r/2.

Optionally, each of the 2ct possible registrations has first and secondcorresponding registration codewords, the first registration codewordsidentifying a first encoding format of the coding pattern and the secondregistration codewords identifying a second encoding format of thecoding pattern.

Optionally, a minimum distance (d_(format)) between each pair of firstand second registration codewords encoding the same registration is atleast r/2.

Optionally, each of the 2ct possible registrations has a correspondingflagged first registration codeword, a corresponding unflagged firstregistration codeword, a corresponding flagged second registrationcodeword and a corresponding unflagged second registration codeword, theflagged registration codewords identifying the presence of an activearea flag and the unflagged registration codewords identifying theabsence of the active area flag.

Optionally, at least some tag-shaped arrays in the coding pattern havingthe first encoding format contain a mixture of registration symbols fromthe flagged first registration codeword and registration symbols fromthe unflagged first registration codeword to form a mixed firstregistration codeword of length r; and

at least some tag-shaped arrays in the coding pattern having the secondformat contain a mixture of registration symbols from the flagged secondregistration codeword and registration symbols from the unflagged secondregistration codeword to form a mixed second registration codeword oflength r.

Optionally, a minimum distance (d_(flag)) between each pair of flaggedand unflagged registration codewords encoding the same registration andthe same format is less than r/2, thereby enabling correction ofregistration symbol errors in the mixed first and second registrationcodewords. Hence, registration and format encoding is prioritized overflag encoding.

In a third aspect, there is provided a method of decoding a codingpattern disposed on or in a substrate, the method comprising the stepsof:

(a) operatively positioning an optical reader relative to a surface ofthe substrate;

(b) capturing an image of a portion of the coding pattern, the codingpattern comprising:

-   -   a tiling of contiguous grid cells, each grid cell being        demarcated by t target elements and having t-fold rotational        symmetry, each grid cell containing nt registration symbols,        each registration symbol being encoded by a set of macrodots;        and    -   a tiling of contiguous tags, each tag consisting of an array of        c grid cells, each tag containing a plurality of data symbols        and having an identical layout of data symbols, each data symbol        being encoded by a set of macrodots;        wherein:

the coding pattern has a physical layout defined by its tiling ofcontiguous grid cells, the physical layout belonging to a plane symmetrygroup that has t-fold rotational symmetry and translational symmetrywith the grid cell as its unit cell;

the coding pattern has a logical layout defined by its tiling ofcontiguous tags, the logical layout belonging to a plane symmetry groupthat has translational symmetry with the tag as its unit cell, but norotational symmetry;

there are ct possible registrations between the physical layout of thecoding pattern and the logical layout of the coding pattern, eachregistration corresponding to a distinct combination of one of the tpossible rotations of the physical layout of the coding pattern relativeto the logical layout of the coding pattern and one of the c possibletranslations of the physical layout of the coding pattern relative tothe logical layout of the coding pattern;

any contiguous tag-shaped array of c grid cells contains cntregistration symbols, the registration symbols, taken in a definedsequence relative to the physical layout of the tag-shaped array,forming a registration codeword of length r;

there are v distinct registration codewords, each corresponding to adistinct one of the ct possible registrations;

the registration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between the tag-shapedarray and the logical layout of the coding pattern;

t=is an integer value of 2 or more;

c=is an integer value of 2 or more;

n is an integer value of 1 or more;

cnt≧r; and

v≧ct;

(c) sampling and decoding at least cnt registration symbols contained inthe imaged portion;

(d) constructing an imaged registration codeword of length r using atleast cnt of the decoded registration symbols ordered in a definedsequence, the defined sequence being determined by positions ofregistration symbols relative to target elements in the imaged portion;

(e) identifying one of v distinct registration codewords correspondingto the imaged registration codeword;

(f) determining a registration corresponding to the identifiedregistration codeword, the registration identifying a registrationbetween a tag-shaped array of c grid cells at least partially containedin the imaged portion and the logical layout of the coding pattern; and

(g) using the identified registration to decode data symbols sampledfrom the imaged portion.

The method according to the third aspect is typically utilized fordecoding the coding pattern defined in connection with the first aspect.

Optionally, a field of view of the optical reader has a diameter ofbetween l and 1.2l, where l is defined as the diameter of one tag.Typically, the field of view is just large enough to contain one wholetag-shaped array of grid cells.

Optionally, the identified registration codeword in step (e) is selectedfrom the v distinct registration codewords on the basis of having asmallest distance from the imaged registration codeword.

Optionally, v=ct and a minimum distance (d_(reg)) between the ctdistinct registration codewords is at least r/2, wherein the methoddetermines the registration in the presence of up to (r/2−1)/2registration symbol errors.

Optionally, v>ct, such that each of the ct possible registrations has aplurality of corresponding registration codewords.

Optionally, a minimum distance between pairs of distinct registrationcodewords encoding different registrations is at least r/2.

Optionally, each of the ct possible registrations has first and secondcorresponding registration codewords, the first registration codewordsidentifying a first encoding format of the coding pattern and the secondregistration codewords identifying a second encoding format of thecoding pattern, and the method comprises the steps of:

identifying one of ct distinct first registration codewords or one of ctdistinct second registration codewords corresponding to the imagedregistration codeword; and

determining a format of the coding pattern using the imaged registrationcodeword.

Optionally, a minimum distance (d_(format)) between each pair of firstand second registration codewords encoding the same registration is atleast r/2, and wherein the method determines the format in the presenceof up to (r/2−1)/2 registration symbol errors.

Optionally, each of the ct possible registrations has a correspondingflagged first registration codeword, a corresponding unflagged firstregistration codeword, a corresponding flagged second registrationcodeword and a corresponding unflagged second registration codeword, theflagged registration codewords identifying the presence of an activearea flag and the unflagged registration codewords identifying theabsence of the active area flag, wherein the method comprises the stepsof;

identifying one of ct distinct flagged first registration codewords, oneof ct distinct unflagged first registration codewords, one of ctdistinct flagged second registration codewords or one of ct distinctunflagged second registration codewords corresponding to the imagedregistration codeword; and

determining the presence or absence of the active area flag using theregistration codeword corresponding to the imaged registration codeword.

Optionally, the active area flag is determined to be present in theevent of any ambiguity in the step of identifying the registrationcodeword corresponding to imaged registration codeword.

Optionally, the imaged registration codeword is a mixed registrationcodeword of length r containing registration symbols from both flaggedand unflagged registration codewords encoding the same format, and themethod comprises the step of:

correcting registration symbol errors in the mixed registrationcodeword, wherein a minimum distance (d_(flag)) between each pair offlagged and unflagged registration codewords encoding the sameregistration and the same format is less than r/2, thereby enablingcorrection of registration symbol errors in the mixed registrationcodeword.

Optionally, each tag comprises at least one local codeword identifying alocation of a respective tag, the local codeword comprising a respectiveset of local data symbols, and wherein the method comprises the step of:

decoding local data symbols contained within the imaged portion todetermine a coordinate location of the optical reader relative to thesurface.

Optionally, each tag comprises one or more common codewords identifyingan identity, each common codeword being common to a plurality ofcontiguous tags, each common codeword comprising a respective set ofcommon data symbols, and wherein the method comprises the step of:

decoding common data symbols contained within the imaged portion todetermine the one or more common codewords; and

determining the identity using the one or more common codewords.

Optionally, the identity identifies at least one of: the substrate; aregion; a page; a product; a visual layout; and an interactivity layout.

In a fourth aspect, there is provided a system for decoding a codingpattern, the system comprising:

(A) a substrate having a coding pattern disposed therein or thereon, thecoding pattern comprising:

-   -   a tiling of contiguous grid cells, each grid cell being        demarcated by t target elements and having t-fold rotational        symmetry, each grid cell containing nt registration symbols,        each registration symbol being encoded by a set of macrodots;        and    -   a tiling of contiguous tags, each tag consisting of an array of        c grid cells, each tag containing a plurality of data symbols        and having an identical layout of data symbols, each data symbol        being encoded by a set of macrodots;        wherein:

the coding pattern has a physical layout defined by its tiling ofcontiguous grid cells, the physical layout belonging to a plane symmetrygroup that has t-fold rotational symmetry and translational symmetrywith the grid cell as its unit cell;

the coding pattern has a logical layout defined by its tiling ofcontiguous tags, the logical layout belonging to a plane symmetry groupthat has translational symmetry with the tag as its unit cell, but norotational symmetry;

there are ct possible registrations between the physical layout of thecoding pattern and the logical layout of the coding pattern, eachregistration corresponding to a distinct combination of one of the tpossible rotations of the physical layout of the coding pattern relativeto the logical layout of the coding pattern and one of the c possibletranslations of the physical layout of the coding pattern relative tothe logical layout of the coding pattern;

any contiguous tag-shaped array of c grid cells contains cntregistration symbols, the registration symbols, taken in a definedsequence relative to the physical layout of the tag-shaped array,forming a registration codeword of length r;

there are v distinct registration codewords, each corresponding to adistinct one of the ct possible registrations;

the registration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between the tag-shapedarray and the logical layout of the coding pattern;

t=is an integer value of 2 or more;

c=is an integer value of 2 or more;

n is an integer value of 1 or more;

cnt≧r; and

v≧ct;

(B) an optical reader comprising:

an image sensor for capturing an image of a portion of the codingpattern; and

a processor configured for performing the steps of:

-   -   (i) sampling and decoding at least cnt registration symbols        contained in the imaged portion;    -   (ii) constructing an imaged registration codeword of length r        using at least cnt of the decoded registration symbols ordered        in a defined sequence, the defined sequence being determined by        positions of registration symbols relative to target elements in        the imaged portion;    -   (iii) identifying one of v distinct registration codewords        corresponding to the imaged registration codeword;    -   (iv) determining a registration corresponding to the identified        registration codeword, the registration identifying a        registration between a tag-shaped array of c grid cells at least        partially contained in the imaged portion and the logical layout        of the coding pattern; and    -   (v) using the identified registration to decode data symbols        sampled from the imaged portion.

In a fifth aspect, there is provided an optical reader for decoding acoding pattern disposed on or in a substrate according to the firstaspect, the optical reader comprising:

an image sensor for capturing an image of a portion of the codingpattern; and

a processor configured for performing the steps of:

-   -   (i) sampling and decoding at least cnt registration symbols        contained in the imaged portion;    -   (ii) constructing an imaged registration codeword of length r        using at least cnt of the decoded registration symbols ordered        in a defined sequence, the defined sequence being determined by        positions of registration symbols relative to target elements in        the imaged portion;    -   (iii) identifying one of v distinct registration codewords        corresponding to the imaged registration codeword;    -   (iv) determining a registration corresponding to the identified        registration codeword, the registration identifying a        registration between a tag-shaped array of c grid cells at least        partially contained in the imaged portion and the logical layout        of the coding pattern; and    -   (v) using the identified registration to decode data symbols        sampled from the imaged portion.

In a sixth aspect, there is provided a method of decoding a codingpattern disposed on or in an at least partially transparent substrate,the method comprising the steps of:

(a) operatively positioning an optical reader relative to either side ofthe substrate;

(b) capturing an image of a portion of the coding pattern, the codingpattern comprising:

-   -   a tiling of contiguous grid cells, each grid cell being        demarcated by t target elements and having t-fold rotational        symmetry, each grid cell containing nt registration symbols,        each registration symbol being encoded by a set of macrodots;        and    -   a tiling of contiguous tags, each tag consisting of an array of        c grid cells, each tag containing a plurality of data symbols        and having an identical layout of data symbols, each data symbol        being encoded by a set of macrodots;        wherein:

the coding pattern has a physical layout defined by its tiling ofcontiguous grid cells, the physical layout belonging to a plane symmetrygroup that has at least one reflection axis, t-fold rotational symmetryand translational symmetry with the grid cell as its unit cell;

the coding pattern has a logical layout defined by its tiling ofcontiguous tags, the logical layout belonging to a plane symmetry groupthat has translational symmetry with the tag as its unit cell, but norotational symmetry and no reflection axes;

there are 2ct possible registrations between the physical layout of thecoding pattern and the logical layout of the coding pattern, eachregistration corresponding to a distinct combination of: (1) whether ornot the physical layout of the coding pattern is reflected relative tothe logical layout of the coding pattern; (2) one of t possiblerotations of the physical layout of coding pattern relative to thelogical layout of the coding pattern; and (3) one of c possibletranslations of the physical layout of the coding pattern relative tothe logical layout of the coding pattern;

any contiguous tag-shaped array of c grid cells contains cntregistration symbols, the registration symbols, taken in a definedsequence relative to the physical layout of the tag-shaped array,forming a registration codeword of length r;

there are v distinct registration codewords, each corresponding to adistinct one of the 2ct possible registrations;

the registration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between the tag-shapedarray and the logical layout of the coding pattern;

t=is an integer value of 2 or more;

c=is an integer value of 2 or more;

n is an integer value of 1 or more;

cnt≧r; and

v≧2ct;

(c) sampling and decoding at least cnt registration symbols contained inthe imaged portion;

(d) constructing an imaged registration codeword of length r using atleast cnt of the decoded registration symbols ordered in a definedsequence, the defined sequence being determined by positions ofregistration symbols relative to target elements in the imaged portion;

(e) identifying one of v distinct registration codewords correspondingto the imaged registration codeword;

(f) determining a registration corresponding to the identifiedregistration codeword; and

(g) using the identified registration to decode data symbols sampledfrom the imaged portion.

In a seventh aspect, there is provided a substrate having a codingpattern disposed thereon or therein, the coding pattern comprising aplurality of macrodots encoding first and second Reed-Solomon datasymbols, wherein:

each first Reed-Solomon data symbol is represented by d macrodots, eachof the d macrodots occupying a respective position from a plurality ofpredetermined possible positions p within a first symbol layout, therespective positions of the d macrodots representing one of a pluralityof possible data values;

each second Reed-Solomon data symbol is represented by d macrodots, eachof the d macrodots occupying a respective position from a plurality ofpredetermined possible positions p within a second symbol layout whichis different than the first symbol layout, the respective positions ofthe d macrodots representing one of a plurality of possible data values;and

p>d.

The coding pattern advantageously uses two different data symbolslayouts so as to allow optimal tessellation of data symbols within thecoding pattern.

Optionally, the first symbol layout and the second symbol layout areeach configured for mutual interlocking.

Optionally, the first symbol layout is substantially L-shaped (in onegiven orientation) and the second symbol layout is substantiallyH-shaped (in one given orientation).

Optionally, the coding pattern comprises contiguous first and secondReed-Solomon data symbols

Optionally, d is an integer value between 2 and 10 (e.g. 2, 3, 4 or 5)and p is an integer value between 4 and 20 (e.g. 5, 6, 7, 8, 9, 10 or11)

Optionally, each of the first and second Reed-Solomon data symbolsprovides i possible symbol values for a j-bit symbol, wherein (i−2^(j))unused symbol values are treated as erasures.

Optionally, the value of d is selected to modify at least one of:

-   -   an overall visibility of the coding pattern; and    -   a data capacity of the coding pattern.

Optionally, the coding pattern further comprises a plurality of targetelements defining a target grid, the targets elements beingdistinguishable from the macrodots.

Optionally, the coding pattern comprises a plurality of symbol groups,each symbol group comprising at least one target element, at least oneregistration symbol and a plurality of the data symbols.

Optionally, the target grid comprises a plurality of grid cells, eachgrid cell having a target element at each corner thereof such thatadjacent grid cells share target elements, and wherein each grid cellcontains one of the symbol groups.

Optionally, the coding pattern comprises a plurality of tags, each tagcomprising a plurality of symbol groups, one or more registrationsymbols and a plurality of target elements.

Optionally, each symbol group comprises a set of the registrationsymbols, the set of registration symbols identifying the integer valueof d.

Optionally, the registration symbols identify one or more of:

a translation of a symbol group relative to a tag containing the symbolgroup, each symbol group containing a plurality of the data symbols;

an orientation of a layout of the data symbols with respect to a targetgrid; and

a flag.

In an eighth second aspect, there is provided a coding pattern disposedthereon or therein, the coding pattern comprising a plurality ofmacrodots encoding a plurality of data symbols, wherein each data symbolis represented by d macrodots, each of the d macrodots occupying arespective position from 7 predetermined possible positions within asymbol layout, the respective positions of the d macrodots representingone of a plurality of possible data values.

The second aspect makes use of multi-bit 7 PPM encoding of data symbols,where a plurality of macrodots occupy different positions within asymbol layout having 7 possible positions. The use of multi-bit PPMencoding provides a more uniform coding pattern compared to simplebinary encoding, which relies on the presence or absence of a macrodotto encode 1 bit of data. Multi-bit PPM encoding also obviates anexternal intensity reference to determine the presence or absence of amacrodot. 7 PPM encoding is optimally used in connection with the codingpattern according to the second aspect.

Optionally, d is 2 or 3.

Optionally, d=3, which provides 35 possible symbol values for a 5-bitdata symbol, wherein 3 unused symbol values are treated as erasures.

Optionally, d=2, which provides 21 possible symbol values for a 4-bitdata symbol, wherein 5 unused symbol values are treated as erasures.

It will appreciated that one or more of the optional embodimentsdescribed herein may be equally applicable to any of the first, second,third, fourth, fifth, sixth, seventh or eighth aspects.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a symbol group of a coding pattern according to the presentinvention;

FIG. 2 is a schematic of a relationship between a sample printed netpageand its online page description;

FIG. 3 shows an embodiment of basic netpage architecture with variousalternatives for the relay device;

FIG. 4 shows the structure of a tag;

FIG. 5 shows the layout of a first 7 PPM data symbol;

FIG. 6 shows the layout of a second 7 PPM data symbol;

FIG. 7 shows the spacing of macrodot positions;

FIG. 8 shows inter-codeword minimum distances in a registration codethat encodes just registration;

FIG. 9 shows inter-codeword minimum distances in a registration codethat encodes registration and two tag format values;

FIG. 10 shows inter-codeword minimum distances in a registration codethat encodes registration, two tag format values and two flag values;

FIG. 11 shows the layout of a registration symbol;

FIG. 12 shows the layout of registration symbols within a symbol group;

FIG. 13 shows the layout of coordinate codewords X and Y, with codewordX shown in bold outline;

FIG. 14 shows the layout of common codewords A, B and C and D;

FIG. 15 shows the layout of a complete tag;

FIG. 16 shows the layout of a Reed-Solomon codeword;

FIG. 17 is a flowchart of image processing;

FIG. 18 shows a nib and elevation of the pen held by a user;

FIG. 19 shows the pen held by a user at a typical incline to a writingsurface;

FIG. 20 is a lateral cross section through the pen;

FIG. 21A is a bottom and nib end partial perspective of the pen;

FIG. 21B is a bottom and nib end partial perspective with the fields ofillumination and field of view of the sensor window shown in dottedoutline;

FIG. 22 is a longitudinal cross section of the pen;

FIG. 23A is a partial longitudinal cross section of the nib and barrelmolding;

FIG. 23B is a partial longitudinal cross section of the IR LED's and thebarrel molding;

FIG. 24 is a ray trace of the pen optics adjacent a sketch of the inkcartridge;

FIG. 25 is a side elevation of the lens;

FIG. 26 is a side elevation of the nib and the field of view of theoptical sensor; and

FIG. 27 is a block diagram of the pen electronics.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS 1.1 NetpageSystem Architecture

In a preferred embodiment, the invention is configured to work with thenetpage networked computer system, a detailed overview of which follows.It will be appreciated that not every implementation will necessarilyembody all or even most of the specific details and extensions discussedbelow in relation to the basic system. However, the system is describedin its most complete form to reduce the need for external reference whenattempting to understand the context in which the preferred embodimentsand aspects of the present invention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper, but which work like interactivewebpages. Information is encoded on each page using ink which issubstantially invisible to the unaided human eye. The ink, however, andthereby the coded data, can be sensed by an optically imaging sensingdevice and transmitted to the netpage system. The sensing device maytake the form of a clicker (for clicking on a specific position on asurface), a pointer having a stylus (for pointing or gesturing on asurface using pointer strokes), or a pen having a marking nib (formarking a surface with ink when pointing, gesturing or writing on thesurface). References herein to “pen” or “netpage pen” are provided byway of example only. It will, of course, be appreciated that the pen maytake the form of any of the sensing devices described above.

In one embodiment, active buttons and hyperlinks on each page can beclicked with the sensing device to request information from the networkor to signal preferences to a network server. In one embodiment, textwritten by hand on a netpage is automatically recognized and convertedto computer text in the netpage system, allowing forms to be filled in.In other embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized. Inother embodiments, text on a netpage may be clicked or gestured toinitiate a search based on keywords indicated by the user.

As illustrated in FIG. 2, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpage 1consists of graphic data 2, printed using visible ink, and a surfacecoding pattern 3 superimposed with the graphic data. The surface codingpattern 3 comprises a collection of tags 4. One such tag 4 is shown inthe shaded region of FIG. 2, although it will be appreciated thatcontiguous tags 4, defined by the coding pattern 3, are densely tiledover the whole netpage 1.

The corresponding page description 5, stored on the netpage network,describes the individual elements of the netpage. In particular itdescribes the type and spatial extent (zone) of each interactive element(i.e. text field or button in the example), to allow the netpage systemto correctly interpret input via the netpage. The submit button 6, forexample, has a zone 7 which corresponds to the spatial extent of thecorresponding graphic 8.

As illustrated in FIG. 3, a netpage sensing device 400, such as the pendescribed in Section 3, works in conjunction with a netpage relay device601, which is an Internet-connected device for home, office or mobileuse. The pen 400 is wireless and communicates securely with the netpagerelay device 601 via a short-range radio link 9. In an alternativeembodiment, the netpage pen 400 utilises a wired connection, such as aUSB or other serial connection, to the relay device 601.

The relay device 601 performs the basic function of relaying interactiondata to a page server 10, which interprets the interaction data. Asshown in FIG. 3, the relay device 601 may, for example, take the form ofa personal computer 601 a, a netpage printer 601 b or some other relay601 c (e.g. personal computer or mobile phone incorporating a webbrowser).

The netpage printer 601 b is able to deliver, periodically or on demand,personalized newspapers, magazines, catalogs, brochures and otherpublications, all printed at high quality as interactive netpages.Unlike a personal computer, the netpage printer is an appliance whichcan be, for example, wall-mounted adjacent to an area where the morningnews is first consumed, such as in a user's kitchen, near a breakfasttable, or near the household's point of departure for the day. It alsocomes in tabletop, desktop, portable and miniature versions. Netpagesprinted on-demand at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

Alternatively, the netpage relay device 601 may be a portable device,such as a mobile phone or PDA, a laptop or desktop computer, or aninformation appliance connected to a shared display, such as a TV. Ifthe relay device 601 is not a netpage printer 601 b which printsnetpages digitally and on demand, the netpages may be printed bytraditional analog printing presses, using such techniques as offsetlithography, flexography, screen printing, relief printing androtogravure, as well as by digital printing presses, using techniquessuch as drop-on-demand inkjet, continuous inkjet, dye transfer, andlaser printing.

As shown in FIG. 3, the netpage sensing device 400 interacts with aportion of the tag pattern on a printed netpage 1, or other printedsubstrate such as a label of a product item 251, and communicates, via ashort-range radio link 9, the interaction to the relay device 601. Therelay 601 sends corresponding interaction data to the relevant netpagepage server 10 for interpretation. Raw data received from the sensingdevice 400 may be relayed directly to the page server 10 as interactiondata. Alternatively, the interaction data may be encoded in the form ofan interaction URI and transmitted to the page server 10 via a user'sweb browser 601 c. The web browser 601 c may then receive a URI from thepage server 10 and access a webpage via a webserver 201. In somecircumstances, the page server 10 may access application computersoftware running on a netpage application server 13.

The netpage relay device 601 can be configured to support any number ofsensing devices, and a sensing device can work with any number ofnetpage relays. In the preferred implementation, each netpage sensingdevice 400 has a unique identifier. This allows each user to maintain adistinct profile with respect to a netpage page server 10 or applicationserver 13.

Digital, on-demand delivery of netpages 1 may be performed by thenetpage printer 601b, which exploits the growing availability ofbroadband Internet access. Netpage publication servers 14 on the netpagenetwork are configured to deliver print-quality publications to netpageprinters. Periodical publications are delivered automatically tosubscribing netpage printers via pointcasting and multicasting Internetprotocols. Personalized publications are filtered and formattedaccording to individual user profiles.

A netpage pen may be registered with a netpage registration server 11and linked to one or more payment card accounts. This allows e-commercepayments to be securely authorized using the netpage pen. The netpageregistration server compares the signature captured by the netpage penwith a previously registered signature, allowing it to authenticate theuser's identity to an e-commerce server. Other biometrics can also beused to verify identity. One version of the netpage pen includesfingerprint scanning, verified in a similar way by the netpageregistration server.

1.2 Netpages

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services.

As shown in FIG. 2, a netpage consists of a printed page (or othersurface region) invisibly tagged with references to an onlinedescription 5 of the page. The online page description 5 is maintainedpersistently by the netpage page server 10. The page descriptiondescribes the visible layout and content of the page, including text,graphics and images. It also describes the input elements on the page,including buttons, hyperlinks, and input fields. A netpage allowsmarkings made with a netpage pen on its surface to be simultaneouslycaptured and processed by the netpage system.

Multiple netpages (for example, those printed by analog printingpresses) can share the same page description. However, to allow inputthrough otherwise identical pages to be distinguished, each netpage maybe assigned a unique page identifier. This page ID has sufficientprecision to distinguish between a very large number of netpages.

Each reference to the page description 5 is repeatedly encoded in thenetpage pattern. Each tag (and/or a collection of contiguous tags)identifies the unique page on which it appears, and thereby indirectlyidentifies the page description 5. Each tag also identifies its ownposition on the page. Characteristics of the tags are described in moredetail below.

Tags are typically printed in infrared-absorptive ink on any substratewhich is infrared-reflective, such as ordinary paper, or in infraredfluorescing ink. Near-infrared wavelengths are invisible to the humaneye but are easily sensed by a solid-state image sensor with anappropriate filter.

A tag is sensed by a 2D area image sensor in the netpage sensing device,and the tag data is transmitted to the netpage system via the nearestnetpage relay device 601. The pen 400 is wireless and communicates withthe netpage relay device 601 via a short-range radio link. It isimportant that the pen recognize the page ID and position on everyinteraction with the page, since the interaction is stateless. Tags areerror-correctably encoded to make them partially tolerant to surfacedamage.

The netpage page server 10 maintains a unique page instance for eachunique printed netpage, allowing it to maintain a distinct set ofuser-supplied values for input fields in the page description 5 for eachprinted netpage 1.

2 Netpage Tags 2.1 Tag Data Content

Each tag 4 identifies an absolute location of that tag within a regionof a substrate.

Each interaction with a netpage should also provide a region identitytogether with the tag location. In a preferred embodiment, the region towhich a tag refers coincides with an entire page, and the region ID istherefore synonymous with the page ID of the page on which the tagappears. In other embodiments, the region to which a tag refers can bean arbitrary subregion of a page or other surface. For example, it cancoincide with the zone of an interactive element, in which case theregion ID can directly identify the interactive element.

As described in the Applicant's previous applications (e.g. U.S. Pat.No. 6,832,717), the region identity may be encoded discretely in eachtag 4. The region identity may be encoded by a plurality of contiguoustags in such a way that every interaction with the substrate stillidentifies the region identity, even if a whole tag is not in the fieldof view of the sensing device.

Each tag 4 should preferably identify an orientation of the tag relativeto the substrate on which the tag is printed. Orientation data read froma tag enables the rotation (yaw) of the pen 101 relative to thesubstrate to be determined

A tag 4 may also encode one or more flags which relate to the region asa whole or to an individual tag. One or more flag bits may, for example,signal a sensing device to provide feedback indicative of a functionassociated with the immediate area of the tag, without the sensingdevice having to refer to a description of the region. A netpage penmay, for example, illuminate an “active area” LED when in the zone of ahyperlink.

A tag 4 may also encode a digital signature or a fragment thereof Tagsencoding (partial) digital signatures are useful in applications whereit is required to verify a product's authenticity. Such applications aredescribed in, for example, US Publication No. 2007/0108285, the contentsof which is herein incorporated by reference. The digital signature maybe encoded in such a way that it can be retrieved from every interactionwith the substrate. Alternatively, the digital signature may be encodedin such a way that it can be assembled from a random or partial scan ofthe substrate.

It will, of course, be appreciated that other types of information (e.g.tag size etc) may also be encoded into each tag or a plurality of tags.

2.2 General Tag Structure

As described above in connection with FIG. 2, the netpage surface codinggenerally consists of a dense planar tiling of tags. In the presentinvention, each tag 4 is defined by a coding pattern which contains twokinds of elements. Referring to FIGS. 1 and 4, the first kind of elementis a target element. Target elements in the form of target dots 301allow a tag 4 to be located in an image of a coded surface, and allowthe perspective distortion of the tag to be inferred. The second kind ofelement is a data element in the form of a dot or macrodot 302 (see FIG.7). Collections of the macrodots 302 encode data values. As described inthe Applicant's earlier disclosures (e.g. U.S. Pat. No. 6,832,717), thepresence or absence of a macrodot was be used to represent a binary bit.However, the tag structure of the present invention encodes a data valueusing multi-pulse position modulation, which is described in more detailin Section 2.3.

The coding pattern 3 is represented on the surface in such a way as toallow it to be acquired by an optical imaging system, and in particularby an optical system with a narrowband response in the near-infrared.The pattern 3 is typically printed onto the surface using a narrowbandnear-infrared ink.

FIG. 4 shows the structure of a complete tag 4 with target elements 301shown. The tag 4 is square and contains sixteen target elements. Thosetarget elements 301 located at the edges and corners of the tag (twelvein total) are shared by adjacent tags and define the perimeter of thetag. The high number of target elements 301 advantageously facilitatesaccurate determination of a perspective distortion of the tag 4 when itis imaged by the sensing device 101. This improves the accuracy of tagsensing and, ultimately, position determination.

The tag 4 consists of a square array of nine symbol groups 303. Symbolgroups 303 are demarcated by the target elements 301 so that each symbolgroup is contained within a square defined by four target elements.Adjacent symbol groups 303 are contiguous and share targets.

Since the target elements 301 are all identical, they do not demarcateone tag from its adjacent tags. Viewed purely at the level of targetelements, only symbol groups 303, which define grid cells of a targetgrid, can be distinguished—the tags 4 themselves are indistinguishableby viewing only the target elements 301. Hence, tags 4 must be alignedwith the target grid as part of tag decoding.

The tag 4 is designed to allow all tag data to be recovered from animaging field of view substantially the size of the tag. This impliesthat any data unique to the tag 4 must appear four times within thetag—i.e. once in each quadrant or quarter; any data unique to a columnor row of tags must appear twice within the tag—i.e. once in eachhorizontal half or vertical half of the tag respectively; and any datacommon to a set of tags needs to appear once within the tag.

2.3 Symbol Groups

As shown in FIG. 1, each of the nine symbol groups 303 contains ten datasymbols 304, each data symbol being part of a codeword. In addition,each symbol group 303 comprises four registration symbols. These allowthe orientation and translation of the tag in the field of view to bedetermined, as well as the tag format to be determined. Translationrefers to the translation of tag(s) relative to the symbol groups 303 inthe field of view. In other words, the registration symbols enablealignment of the ‘invisible’ tags with the target grid.

Each data symbol 304 is a multi-pulse position modulated (PPM) datasymbol. Each PPM data symbol 304 encodes a single 4-bit or 5-bitReed-Solomon symbol using 2 or 3 macrodots in any of 7 positions{p₀,p₁,p₂,p₃,p₄,p₅,p₆}, i.e. using 2-7 or 3-7 pulse-position modulation(PPM). 2-7 PPM is used if the tag format is 0; 3-7 PPM is used if thetag format is 1. 3-7 PPM has a range of 35 symbol values enabling 5-bitencoding with 3 unused symbol values, while 2-7 PPM has a range of 21values enabling 4-bit encoding with 5 unused symbol values.

FIG. 5 shows the layout for a first 7 PPM data symbol 304A, which issubstantially L-shaped. FIG. 6 shows the layout for a second 7 PPM datasymbol 304B, which is substantially H-shaped. The differently shaped 7PPM data symbols 304A and 304B allow optimal tessellation of the datasymbols when they interlock to form the coding pattern 3.

Table 1 defines the mapping from 2-7 PPM symbol values to Reed-Solomondata symbol values. Unused symbol values may be treated as erasures.

TABLE 1 2-7PPM symbol to 4-bit data symbol value mapping 2-7PPM symbolvalue 4-bit data symbol value (p₆-p₀) (base 16) 0,000,011 unused0,000,101 0 0,000,110 unused 0,001,010 1 0,001,010 2 0,001,100 30,010,001 4 0,010,010 5 0,010,100 unused 0,011,000 6 0,100,001 70,100,010 8 0,100,100 9 0,101,000 a 0,110,000 b 1,000,001 c 1,000,010 d1,000,100 e 1,001,000 f 1,010,000 unused 1,100,000 unused

Unused PPM symbol values are chosen to avoid macrodot pairs along theconvex edge of the first L-shaped 7 PPM data symbol 304A shown in FIG.5, in order to avoid clustering or clumping of macrodots 302 betweenadjacent data symbols. Thus, the (p₀, p₁), (p₁, p₂), (p₂, p₄), (p₄, p₆)and (p₅, p₆) doublets are unused in 2-7 PPM encoding because thesedoublets are positioned along the convex edge of the first data symbol304A. With the tessellated tiling of data symbols 304 in the codingpattern, this non-arbitrary use of unused symbol values helps tominimize visibility of the coding pattern by maintaining a more evendistribution of macrodots.

Table 2 defines the mapping from 3-7 PPM symbol values to data symbolvalues. Unused symbol values may be treated as erasures

TABLE 2 3-7PPM symbol to 5-bit data symbol value mapping 3-7PPM symbolvalue 5-bit data symbol value (p₆-p₀) (base 16) 0,000,111  0 0,001,011unused 0,001,101  1 0,001,110  2 0,010,011  3 0,010,101  4 0,010,110unused 0,011,001  5 0,011,010  6 0,011,100  7 0,100,011  8 0,100,101  90,100,110 a 0,101,001 b 0,101,010 c 0,101,100 d 0,110,001 e 0,110,010 f0,110,100 10 0,111,000 11 1,000,011 12 1,000,101 13 1,000,110 141,001,001 15 1,001,010 16 1,001,100 17 1,010,001 18 1,010,010 191,010,100 1a 1,011,000 1b 1,100,001 1c 1,100,010 1d 1,100,100 1e1,101,000 1f 1,110,000 unused

Unused PPM symbol values are chosen to avoid macrodot triplets at thecorners of the first 7 PPM data symbol 304A shown in FIG. 1, in order toavoid clustering or clumping of macrodots 302 between adjacent datasymbols 304. Thus, the (p₀, p₁, p₃), (p₁, p₂, p₄) and (p₄, p₅, p₆)triplets are unused in 3-7 PPM encoding because these triplets arepositioned at the corners of the first data symbol 304A. With thetessellated tiling of data symbols 304 in the coding pattern, thisnon-arbitrary use of unused symbol values helps to minimize visibilityof the coding pattern by maintaining a more even distribution ofmacrodots.

2.4 Targets and Macrodots

The spacing of macrodots 302 in both dimensions, as shown in FIG. 7, isspecified by the parameter s. It has a nominal value of 127 μm, based on8 dots printed at a pitch of 1600 dots per inch.

Only the macrodots 302 are part of the representation of a data symbol304 in the coding pattern. The outline of a symbol 304 is shown in, forexample, FIGS. 1 and 4 merely to elucidate more clearly the structure ofa tag 4.

A macrodot 302 is nominally round with a nominal size of (⅝)s. However,it is allowed to vary in size by ±10% according to the capabilities ofthe device used to produce the pattern.

A target 301 is nominally circular with a nominal diameter of ( 12/8)s.However, it is allowed to vary in size by ±10% according to thecapabilities of the device used to produce the pattern.

Each tag 4 has a width of 30s and a length of 30s. However, it should benoted from FIG. 4 that the tag 4 is configured so that some data symbolsextend beyond the perimeter edge of the tag 4 and interlock withcomplementary symbol groups from adjacent tags. This arrangementprovides a tessellated pattern of data symbols 304 within the codingpattern 3.

The macrodot spacing, and therefore the overall scale of the tagpattern, is allowed to vary by 120 μm and 127 μm according to thecapabilities of the device used to produce the pattern. Any deviationfrom the nominal scale is recorded in each tag (via a macrodot size IDfield) to allow accurate generation of position samples.

These tolerances are independent of one another. They may be refinedwith reference to particular printer characteristics.

2.5 Field of View

As mentioned above, the tag 4 is designed to allow all tag data to berecovered from an imaging field of view roughly the size of the tag. Anydata common to a set of contiguous tags only needs to appear once withineach tag, since fragments of the common data can be recovered fromadjacent tags. Any data common only to a column or row of tags mayappear twice within the tag—i.e. once in each horizontal half orvertical half of the tag respectively. Any data unique to the tag mustappear four times within the tag—i.e. once in each quadrant.

Although data which is common to a set of tags, in one or both spatialdimensions, may be decoded from fragments from adjacent tags,pulse-position modulated values are best decoded from spatially-coherentsamples (i.e. from a whole symbol as opposed to partial symbols atopposite sides of the field of view), since this allows raw samplevalues to be compared without first being normalised. This implies thatthe field of view must be large enough to contain two complete copies ofeach such pulse-position modulated value.

The tag is designed so that the maximum extent of a pulse-positionmodulated value is three macrodots (see FIG. 4). Thus, the minimumimaging field of view required to guarantee acquisition of an entire taghas a diameter of 46.7s (i.e. (30+3)√2s), allowing for arbitraryrotation and translation of the surface coding in the field of view.This field of view has a diameter of one tag plus one data symbol. Thisextra data symbol ensures that PPM data symbols can be decoded fromcontiguous macrodots.

Given a maximum macrodot spacing of 127 μm, the minimum required fieldof view has a diameter of 5.93 mm.

2.6 Encoded Codes and Codewords

In this following section (Section 2.6), each symbol in FIGS. 8 to 12 isshown with a unique label. The label consists of an alphabetic prefixwhich identifies which codeword the symbol is part of, and a numericsuffix which indicates the index of the symbol within the codeword. Theorientation of each symbol label indicates the orientation of thecorresponding symbol layout and is consistent with the symbolorientations shown in FIG. 1.

2.6.1 Designing an Optimized Registration Code

As discussed above, each tag 4 contains an array of square symbol groups303, with each symbol group being demarcated by a set of target elements301. In the present coding pattern 3 (known as “Cassia”), each tag 4contains an array of 3×3 symbol groups with each symbol group beingdemarcated by four target elements. However, it will be appreciated thatother configurations are of course possible e.g. 2×2, 4×4 etc.

The physical layout of the surface coding pattern consists of targetelements which define ‘grid cells’ of a target grid. Each grid cell ofthe target grid, when viewed purely at the level of target elements andmacrodot regions, may be described as a ‘physical symbol group’. Thephysical layout of the coding pattern is designed so that it belongs toa plane symmetry group that has rotational symmetry of the same order asthe number of targets delineating its physical symbol groups i.e.four-fold rotational symmetry. The physical layout also hastranslational symmetry with the physical symbol group as its unit cell.The present coding pattern has a physical layout which belongs tosymmetry group p4 m. However, 4-fold rotational symmetry is not anessential requirement of coding patterns utilizing the presentregistration encoding. In alternative coding patterns, the physicallayout may be 2-fold, 3-fold or 6-fold rotationally symmetric. Examplesof such physical layouts are described in U.S. Pat. No. 7,111,791, thecontents of which is herein incorporated by reference.

However, the logical layout of a surface coding pattern (i.e. consistingof arrangements of data symbols and codewords—see FIG. 12) no longer hasthe same four-fold rotational symmetry as the physical layout. Rather,the logical layout is designed so that it belongs to a plane symmetrygroup that has translational symmetry with the tag, rather than thesymbol group, as its unit cell. The logical layout of the present codingpattern has the tag as its unit cell and a logical layout which belongsto symmetry group p1.

During decoding, the physical layout (i.e. the layout of 4-foldrotationally symmetric cells) is directly discernable from the physicalpattern of the targets 301. However, the rotation and translation of thelogical layout with respect to the physical layout must be determined bythe decoding process.

Assuming t targets per physical symbol group and c physical symbolgroups per tag, there are ct possible transformations (or registrations)between a physical layout and a logical layout. Thus, the present codingpattern has 4×3×3=36 possible registrations.

The Netpage surface coding includes a registration code that allows theregistration between the physical layout and the logical layout to bedetermined The physical layout of the registration code has the samesymmetry as the physical layout of the surface coding itself, while thelogical layout of the registration code has the same symmetry as thelogical layout of the surface coding. Crucially, however, theregistration code is designed so that a valid codeword of theregistration code can be read according to just the physical layout ofthe registration code, i.e. with an arbitrary registration, and thiscodeword will uniquely indicate the actual registration.

In the present coding pattern, the registration code contains oneregistration symbol per target within a symbol group, i.e. tregistration symbols per symbol group and therefore ct registrationsymbols per tag. Hence, the present coding pattern contains 36registration symbols. However, the number of registration symbols withineach symbol group may be any integer multiple n of the rotationalsymmetry. For example, in the 4-fold rotationally symmetric symbolgroups described herein, each symbol group may contain 4, 8, 12, 16 etcregistration symbols positioned in a 4-fold rotationally symmetricarrangement.

In addition to registration, it may be convenient to encode otherinformation in the registration code. For example, if a surface codingsupports multiple levels of redundancy and/or multiple modulationschemes for its data content, then the registration code can be used toencode the format of the tag data. It is also convenient to encode anactive area flag that indicates whether a particular tag is part of anactive area on the surface.

Since a surface coding is designed to be decodable from an imaging fieldof view (FOV) just large enough to contain a single tag, a decodergenerally utilizes fragments from adjacent tags. While the tag format iscommon to an entire surface region, the active area flag varies by tag.The active area flag is therefore more difficult to encode effectivelyin a registration code.

When the registration code has sufficient capacity it is convenient toencode registration, format and flag using a structured scheme that iseasy to decode. In the structured scheme, each symbol group contains aregistration code which independently identifies the registration,format and flag. However, when the registration code does not havesufficient capacity for a structured scheme it is convenient to use anunstructured scheme that can be optimized for the available capacity.The present invention employs an unstructured code, which has theadvantage of minimizing the data capacity of registration symbols.

The capacity of a registration code is a function of both its length andthe capacity of its symbols. Since its length is proportional to thenumber of registrations it must encode, the capacity of its symbolsdetermines its effective capacity.

Using d -in- p multi-pulse position modulation, the capacity C of asymbol is:

C=p!/(p−d)!d!

Thus, registration symbols using 2-5 PPM have a capacity of 10 values,while reduced capacity registration symbols according to the presentinvention using 2-4 PPM have a capacity of 6 values (see Table 3 inSection 2.6.2).

An unstructured registration code is optimal in the sense that itserror-correcting capacity is maximal. Its error-correcting capacity ishigher than that of a structured registration code, but at the expenseof a more complex decoder. The decoder for a structured code typicallyconsists of a set of minimum-distance decoders operating on shortersub-codes within the code, while the decoder for an unstructured codetypically consists of a single minimum-distance decoder operating on theentire code.

A registration code that encodes just registration consists of ctcodewords, each of which corresponds to one of the possibletransformations of a single base codeword, as illustrated in FIG. 8. (InFIG. 8, the dotted arrow indicates distances between differentregistrations of the base codeword, with all distances being equal orexceeding d_(min)).

An optimal registration code is obtained by maximising the minimumdistance between its codewords.

No analytic method for obtaining an optimal registration code of acertain size and symbol capacity, nor even for determining its maximalminimum distance, is known to the present Applicant. The vector space ofpossible base codewords has a size of C^(ct). For typical code sizes andsymbol capacities this space is not amenable to exhaustive search.Useful registration codes can, however, be obtained by random search.

A registration code that encodes both registration and f possible tagformat values consists of ctf codewords, each of which corresponds toone of the ctf possible transformations off base codewords, asillustrated in FIG. 9 (for f=2). (In FIG. 9, the dotted arrows indicatedistances between different registrations of base codewords, with solidarrows indicating the distance between base codewords and all distancesbeing equal or exceeding d_(min)).

An optimal format-encoding registration code is obtained by maximisingthe minimum distance between its codewords. A registration code thatencodes registration, f possible tag format values, and g possible flagvalues consists of ctfg codewords, each of which corresponds to one ofthe possible transformations of fg base codewords, as illustrated inFIG. 10 (for f=2 and g=2). (In FIG. 10, the dotted arrows indicatedistances between different registrations of base codewords, with solidarrows indicating the distance between base codwords and all distancesbeing equal or exceeding d_(min)).

Because the flag value varies by tag, an optimal flag-encodingregistration code is no longer obtained simply by maximising the minimumdistance between its codewords. Instead, because a decoder needs to beable to decode the registration code from fragments from adjacent tags,the registration code needs to be designed so that any variation in flagvalue between adjacent tags does not excessively undermine registrationdecoding by introducing apparent errors.

The impact of flag value variation on the robustness of registrationdecoding can be reduced, at the expense of the robustness of flagdecoding, by selecting the base codewords so that the distance betweenbase codewords representing the same format value but different flagvalues is shorter than the minimum distance otherwise. This relativelyshorter distance between different flag base codewords is referred to asd_(flag), and is labelled as such in FIG. 10. In all other cases theminimum distance is relatively longer.

In practice, relatively less robust flag decoding is usually notproblematic. The flag is typically interpreted as a hint and cantherefore safely be interpreted as set if its value is ambiguous.

The registration code may additionally allow a reflection of the logicallayout relative to the physical layout to be determined. The physicallayout has at least one 2D reflection axis so that the physical layoutis preserved in its mirror image, while the logical layout has noreflection axes. A modified registration code, which allows reflectionto be determined, is useful if the coding pattern is imaged inreflection, e.g. reflected in a mirror or through the back of atransparent substrate.

With at least one reflection axis per physical symbol group, t targetsper physical symbol group and c physical symbol groups per tag, therewill be 2ct possible registrations between a physical layout and alogical layout. Therefore, in this modified registration code, there are2×4×3×3=72 possible registrations.

Although the values of the 2-4 PPM registration symbols will bedifferent depending on whether they are read from the front or back ofthe substrate (i.e. reflected or not), this can be readily taken intoaccount when the code is optimized so as to maximally separate itscodewords. Accordingly, the differences in registration symbol values isnot an issue when reading the coding pattern from the front or back ofthe substrate.

2.6.2 Optimized Unstructured Registration Code

Each symbol group comprises four registration symbols, nominallydesignated R0, R1, R2, and R3. Each registration symbol 307 is encodedusing 2-4 PPM and so is limited to 6 symbol values. FIG. 11 shows thelayout of one 4 PPM registration symbol 307

As shown in FIG. 1 and FIG. 12, the registration symbols 307 each appearfour times within a symbol group.

Table 3 defines the mapping from 2-4 PPM symbol values to registrationsymbol values. Unused symbol values are treated as erasures.

TABLE 3 2-4PPM symbol to registration symbol mapping 2-4PPM symbol valueregistration symbol (p₃-p₀) value 0011 0 0101 1 0110 2 1001 3 1010 41100 5

The registration code is defined by four base codewords, each with 36variants corresponding to the 36 possible registrations between a symbolgroup 303 and a tag 4. The registration code therefore includes 144codewords. (Each of the four base codewords would have 72 variants ifthe registration code allowed reflections to be determined, as describedin Section 2.6.1.)

A tag 4 contains a single base registration codeword consisting of 4registration symbols in each symbol group, i.e. 36 registration symbolsin all, as shown in FIG. 1 and FIG. 12.

The registration codeword of a tag encodes the orientation, translation,format and active area flag of the tag. Table 4 defines the four baseregistration codewords, each of which encodes one of the fourcombinations of tag format and active area flag.

TABLE 4 Base registration codewords active tag area codeword format flag4414, 0332, 2253, 1132, 1411, 3402, 5503, 2113, 2155 0 0 2414, 3332,4253, 4132, 4411, 5402, 4503, 1113, 3155 1 2321, 5555, 1101, 1212, 5500,4540, 5531, 4311, 4512 1 0 0321, 3555, 0101, 0212, 4500, 5540, 1531,5311, 5512 1

Each base codeword encodes zero rotation and translation, i.e. it isonly when a base codeword is read at a non-zero rotation and/ortranslation that the resulting variant of the base codeword encodes thecorresponding non-zero rotation and/or translation.

A base codeword is assigned to the registration symbols of a tagstarting with the left-most symbol of the codeword being assigned to R0of the top left symbol group of a tag, and then proceeding left-to-rightwithin the codeword and column-wise to the right and row-wise down inthe symbol group array of the tag.

The registration code is designed so that it has a minimum distance of26, with the exception of pairs of codewords that encode the sameregistration and tag format but different active area flags. Thedistance within these pairs (d_(flag)) is 9. This ensures that localvariations in the value of the active area flag do not destroy theerror-correcting capacity of the registration code.

A registration codeword is decoded using a minimum distance decoder. Inthe absence of active area flag variation the registration code has thecapacity to correct 12 errors. In the presence of active area flagvariation it has a worst-case capacity to correct 8 errors.

2.6.3 Coordinate Data

The tag contains an x-coordinate codeword and a y-coordinate codewordused to encode the x and y coordinates of the tag respectively. Thecodewords are of a shortened 2⁴-ary or 2⁵-ary (9, 3) Reed-Solomon code.The tag therefore encodes either two 12-bit or two 15-bit coordinates. A2⁴-ary code is used if the tag format is 0; a 2⁵-ary code is used if thetag format is 1.

Each x coordinate codeword is replicated twice within the tag—in eachhorizontal half (“north” and “south”), and is constant within the columnof tags containing the tag. Likewise, each y coordinate codeword isreplicated twice within the tag—in each vertical half (“east” and“west”), and is constant within the row of tags containing the tag. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of eachcoordinate codeword, irrespective of the alignment of the image with thetag pattern. The instance of either coordinate codeword may consist offragments from different tags.

It should be noted that some coordinate symbols are not replicated andare placed on the dividing line between the two halves of the tag. Thisarrangement saves tag space since there are not two completereplications of each x-coordinate codeword and each y-coordinatecodeword contained in a tag. Since the field of view is at least threemacrodot units larger than the tag (as discussed in Section 2.5), thecoordinate symbols placed on the dividing line (having a width 3macrodot units) are still captured when the surface is imaged. Hence,each interaction with the coded surface still provides the tag location.

The layout of the x-coordinate codeword and y-coordinate codeword isshown in FIG. 13. The coordinate codewords have the same layout, rotated90 degrees relative to each other. It can be seen that x-coordinatesymbols X0, X1 and X2 are placed in a central column 310 of the tag 4,which divides the eastern half of the tag from the western halfLikewise, the y-coordinate symbols Y0, Y1 and Y2 are placed in a centralrow 312 of the tag 4, which divides the northern half of the tag fromthe southern half

The number of non-replicated coordinate data symbols appearing in thecentral column and central row of the tag 4 is minimized in thecoordinate symbol layout shown in FIG. 13. By minimizing the number ofnon-replicated coordinate data symbols in one line, the effects of anyinterference from co-printed visible lines (e.g. horizontal and verticallines of a field form box) are minimized Accordingly, the arrangement ofcoordinate data symbols shown in FIG. 13 optimizes tag reading withother co-printed information.

The central column 310 and central row 312 each have a width q, whichcorresponds to a width of 3s, where s is the macrodot spacing.

2.6.4 Common Data

The tag contains three codewords A, B, C and D which encode informationcommon to a set of contiguous tags in a surface region. The codewordsare of a 2⁴-ary or shortened 2⁵-ary (15, 7) Reed-Solomon code. The tagtherefore encodes either 112 bits or 140 bits of information common to aset of contiguous tags. A 2⁴-ary code is used if the tag format is 0; a2⁵-ary code is used if the tag format is 1.

The common codewords are replicated throughout a tagged region. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of each commoncodeword, irrespective of the alignment of the image with the tagpattern. The instance of each common codeword may consist of fragmentsfrom different tags.

The layout of the common codewords is shown in FIG. 14. The codewordshave the same layout, rotated 90 degrees relative to each other.

2.6.5 Complete Tag

FIG. 15 shows the layout of the data of a complete tag, including datasymbols and registration symbols.

2.7 Error Detection and Correction 2.7.1 Reed-Solomon Encoding

All data is encoded using a Reed-Solomon code defined over GF(2^(m)),where the degree m=4 when the tag format is 0, and m=5 when the tagformat is 1.

The code has a natural length n of 2^(m)−1. The dimension k of the codeis chosen to balance the error correcting capacity and data capacity ofthe code, which are (n−k)/2 and k symbols respectively.

The code may be punctured, by removing high-order redundancy symbols, toobtain a code with reduced length and reduced error correcting capacity.The code may also be shortened, by replacing high-order data symbolswith zeros, to obtain a code with reduced length and reduced datacapacity. Both puncturing and shortening can be used to obtain a codewith particular parameters. Shortening is preferred, where possible,since this avoids the need for erasure decoding.

The code has the following primitive polynominals:

p(x)=x ⁴ +x+1, when m=4

p(x)=x ⁵ +x ²+1, when m=5

The code has the following generator polynominal:

${g(x)} = {\prod\limits_{i = 1}^{n - k}\; \left( {x + \alpha^{l}} \right)}$

For a detailed description of Reed-Solomon codes, refer to Wicker, S. B.and V. K. Bhargava, eds., Reed-Solomon Codes and Their Applications,IEEE Press, 1994.

2.7.2 Codeword Organization

As shown in FIG. 16, redundancy coordinates r_(i) and data coordinatesd_(i) of the code are indexed from left to right according to the powerof their corresponding polynomial terms. The symbols X_(i) of a completecodeword are indexed from right to left to match the bit order of thedata. The bit order within each symbol is the same as the overall bitorder.

2.7.3 Code Instances

Table 5 defines the parameters of the different codes used in the tag.

TABLE 5 Codeword instances error-correcting data codeword codewordlength dimension capacity degree capacity^(a) name description (n) (k)(symbols) (m) (bits) X, Y coordinate codewords 9 3 3 4 12 (see Section2.6.3) 5 15 A, B, C, D common codewords 15 7 4 4 28 (see Section 2.6.4)5 35

2.7.4 Cyclic Redundancy Check

The region ID encoded by the common codewords is protected by a 16-bitcyclic redundancy check (CRC). This provides an added layer of errordetection after Reed-Solomon error correction, in case a codewordcontaining a part of the region ID is mis-corrected.

The CRC has the following generator polynomial:

g(x)=x ¹⁶ +x ¹² +x ⁵+1

The CRC is initialised to 0xFFFF. The most significant bit of the regionID is treated as the most significant coefficient of the datapolynomial.

2.8 Tag Coordinate Space

The tag coordinate space has two orthogonal axes labelled x and yrespectively. When the positive x axis points to the right then thepositive y axis points down.

The surface coding does not specify the location of the tag coordinatespace origin on a particular tagged surface, nor the orientation of thetag coordinate space with respect to the surface. This information isapplication-specific. For example, if the tagged surface is a sheet ofpaper, then the application which prints the tags onto the paper mayrecord the actual offset and orientation, and these can be used tonormalise any digital ink subsequently captured in conjunction with thesurface.

The position encoded in a tag is defined in units of tags and is definedto be the centre of the top left target. The origin of a particular tagpattern is therefore the centre of the top left target of the tag thatencodes coordinate pair (0, 0).

The surface coding is optionally displaced from its nominal positionrelative to the surface by an amount derived from the region ID. Thisensures that the utilisation of a pagewidth digital printhead used toprint the surface coding is uniform. The displacement of the surfacecoding is negative, hence the displacement of the region described bythe surface coding is positive relative to the surface coding. Themagnitude of the displacement is the region ID modulo the width of thetag in 1600 dpi dots (i.e. 240). To accommodate non-1600 dpi printersthe actual magnitude of the displacement may vary from its nominal valueby up to half the dot pitch of the printer.

2.9 Tag Information Content 2.9.1 Field Definitions

Table 6 defines the information fields embedded in the surface coding.

TABLE 6 Field Definitions width field (bits) description unique to tagactive area flag 1 A flag indicating whether the area^(a) immediatelysurrounding a tag intersects an active area. x coordinate 12 or 15 Theunsigned x coordinate of the tag^(b). y coordinate 12 or 15 The unsignedy coordinate of the tag^(b). common to tagged region tag format 1 Theformat of the tag: 0: 2-7PPM, m = 4 1: 3-7PPM, m = 5 encoding format 2The format of the encoding. 0: the present encoding. Other values arereserved region flags 10  Flags controlling the interpretation of regiondata (see Table 7). macrodot size ID 4 The ID of the macrodot size.region ID 80 or 108 The ID of the region containing the tags. CRC(Cyclic 16  A CRC of the region ID Redundancy (see Section 2.7.4).Check) ^(a)the diameter of the area, centered on the tag, is nominally2.5 times the diagonal size of the tag; this is to accommodate theworst-case distance between the nib position and the imaged tag^(b)allows a coordinate value ranges of 15 m and 118 km respectively forthe minimum tag size of 3.6 mm (based on the minimum macrodot size of120 microns and 30 macrodots per tag)

An active area is an area within which any captured input should beimmediately forwarded to the corresponding Netpage server 10 forinterpretation. This also allows the Netpage server 10 to signal to theuser that the input has had an immediate effect. Since the server hasaccess to precise region definitions, any active area indication in thesurface coding can be imprecise so long as it is inclusive.

TABLE 7 Region flags bit meaning 0 Region is interactive, i.e. x andy-coordinates are present. 1 Region is active, i.e. the entire region isan active area. Otherwise active areas are identified by individualtags' active area flags. 2 Region ID is serialized^(a). 3 Region IDcontains a digital signature^(b) 4 Region ID is an EPC 5 Region isdisplaced according to region ID (see Section 2.8) other Reserved forfuture use ^(a)If not set for an EPC this means that the serial numberis replaced by a layout number, to allow the package design associatedwith a product to vary over time (see US 2007/0108285, the contents ofwhich is herein incorporated by reference). ^(b)Hence the region IDshould not be transmitted in the clear during resolution.

2.9.2 Mapping of Fields to Codewords

Table 8 and Table 9 define how the information fields map to codewords.

TABLE 8 Mapping of fields to coordinate codewords X and Y codewordcodeword field tag format field width field bits bits X x coordinate 012 all all 1 15 Y y coordinate 0 12 all all 1 15

TABLE 9 Mapping of fields to common codewords A, B, C and D fieldcodeword codeword field tag format width field bits bits A CRC any 16all 15:0  region ID 0 12 11:0  27:16 1 19 18:0  34:16 B encoding formatany 2 all 1:0 region flags any 10 all 11:2  macrodot size ID any 4 all15:12 region ID 0 12 23:12 27:16 1 19 37:19 34:16 C region ID 0 28 51:24all 1 35 72:38 all D region ID 0 28 79:52 all 1 35 107:73  all

When the region flags indicate that a particular codeword is absent thenthe codeword is not coded in the tag pattern, i.e. there are nomacrodots representing the codeword. This applies to the X and Y i.e.the X and Y codewords are present if the <region is interactive> flag inthe region flags is set.

2.10 Tag Imaging and Decoding

As explained above, the minimum imaging field of view required toguarantee acquisition of data from an entire tag has a diameter of 47.6s(i.e. (30+3)√2s), allowing for arbitrary rotation and translation of thesurface coding in the field of view. Notably, the imaging field of viewdoes not have to be large enough to guarantee capture of an entiretag—the arrangement of the data symbols within each tag ensures that aany square portion of length (l+3s) captures the requisite informationin full, irrespective of whether a whole tag is actually visible in thefield-of-view. As used herein, l is defined as the length of a tag.

The extra three macrodot units ensure that pulse-position modulatedvalues can be decoded from spatially coherent samples. Furthermore, theextra three macrodot units ensure that all requisite data symbols can beread with each interaction. These include the coordinate symbols from acentral column or row of a tag (see Section 2.6.3) having a width of 3s.

In the present context, a “tag diameter” is given to mean the length ofa tag diagonal. FIG. 17 shows a tag image processing and decodingprocess flow up to the stage of sampling and decoding the datacodewords. Firstly, a raw image 802 of the tag pattern is acquired (at800), for example via an image sensor such as a CCD image sensor, CMOSimage sensor, or a scanning laser and photodiode image sensor. The rawimage 802 is then typically enhanced (at 804) to produce an enhancedimage 806 with improved contrast and more uniform pixel intensities.Image enhancement may include global or local range expansion,equalisation, and the like. The enhanced image 806 is then typicallyfiltered (at 808) to produce a filtered image 810. Image filtering mayconsist of low-pass filtering, with the low-pass filter kernel sizetuned to obscure macrodots 302 but to preserve targets 301. Thefiltering step 808 may include additional filtering (such as edgedetection) to enhance target features 301. Encoding of data codewords304 using pulse position modulation (PPM) provides a more uniform codingpattern 3 than simple binary dot encoding (as described in, for example,U.S. Pat. No. 6,832,717). Advantageously, this helps separate targets301 from data areas, thereby allowing more effective low-pass filteringof the PPM-encoded data compared to binary-coded data.

Following low-pass filtering, the filtered image 810 is then processed(at 812) to locate the targets 301. This may consist of a search fortarget features whose spatial inter-relationship is consistent with theknown geometry of the tag pattern. Candidate targets may be identifieddirectly from maxima in the filtered image 810, or may be the subject offurther characterization and matching, such as via their (binary orgrayscale) shape moments (typically computed from pixels in the enhancedimage 806 based on local maxima in the filtered image 810), as describedin U.S. Pat. No. 7,055,739, the contents of which is herein incorporatedby reference.

The identified targets 301 are then assigned (at 816) to a target grid818. At this stage, individual tags 4 will not be identifiable in thetarget grid 818, because the targets 301 do not demarcate one tag fromanother.

To allow macrodot values to be sampled accurately, the perspectivetransform of the captured image must be inferred. Four of the targets301 are taken to be the perspective-distorted corners of a square ofknown size in tag space, and the eight-degree-of-freedom perspectivetransform 822 is inferred (at 820), based on solving the well-understoodequations relating the four tag-space and image-space point pairs.Calculation of the 2D perspective transform is described in detail in,for example, Applicant's U.S. Pat. No. 6,832,717, the contents of whichis herein incorporated by reference.

The inferred tag-space to image-space perspective transform 822 is usedto project each known macrodot position in tag space into image space.Since all bits in the tags are represented by PPM-encoding, the presenceor absence of each macrodot 302 can be determined using a localintensity reference, rather than a separate intensity reference. Thus,PPM-encoding provides improved data sampling compared with pure binaryencoding.

At the next stage, at least 36 registration symbols visible within thefield of view are sampled and decoded (at 824). Decoding of the 36registration symbols enables to the pen to construct a sampledregistration codeword consisting of 36 registration symbol valuesarranged in a sequence determined by reading each registration symbolwithin the field of view in a predetermined order.

The sampled registration codeword is identified as a variant of one ofthe four base registration codewords defined in Table 4. As discussedabove in Section 2.6.2, identification of one of the four baseregistration codewords enables the tag format code (0 or 1) and flagcode (0 or 1) to be determined (at 830). Moreover, the particularvariant of the base registration codeword corresponding to the sampledregistration codeword enables the registration (i.e. the verticaltranslation, horizontal translation and orientation) to be determined(at 830).

The horizontal and vertical translations (identified from the sampledregistration codeword) are used to determine the translation of tags(s)(or portions thereof) in the field of view relative to the target grid818. This enables alignment of the tags 4 with the target grid 818,thereby allowing individual tag(s), or portions thereof, to bedistinguished in the coding pattern 3 in the field of view. Usually, awhole tag will not be visible in the field of view although it is, ofcourse, possible for a whole tag to be visible when the field of view isaligned with the coding pattern at zero horizontal and verticaltranslations.

The orientation (identified from the sampled registration codeword) isused to determine the orientation of the data symbols relative to thetarget grid 818.

The flag code is used to identify the presence of absence of an activearea flag in the tag (or part thereof) within the field of view.

The tag format code is used to determine (at 825) the data symbolmodulation. The tag format codes of 0 and 1 identify 2-7 PPM and 3-7 PPMencoding of data symbols, respectively (at 826). The type of PPMencoding needs to be known before decoding of the data symbols (at 836).

Once initial imaging and decoding has yielded the 2D perspectivetransform, the orientation, the translation of tag(s) relative to thetarget grid and the format of the data symbols, the data codewords 304can then be sampled and decoded (at 836) to yield the requisite decodedcodewords 838.

Decoding of the data codewords 304 typically proceeds as follows:

-   -   sample and decode Reed-Solomon codeword containing common data        (A, B, C and D)    -   verify CRC of common data    -   on decode error flag bad region ID sample    -   determine region ID    -   sample and decode x and y coordinate Reed-Solomon codewords (X        and Y)    -   determine tag x-y location from codewords    -   determine nib x-y location from tag x-y location and perspective        transform taking into account macrodot size (from macrodot size        ID)    -   determine active area status of nib location with reference to        active area flag identified from registration code    -   encode region ID, nib x-y location, and nib active area status        in digital ink (“interaction data”)

In practice, when decoding a sequence of images of a tag pattern, it isuseful to exploit inter-frame coherence to obtain greater effectiveredundancy.

Region ID decoding need not occur at the same rate as position decoding.

The skilled person will appreciate that the decoding sequence describedabove represents one embodiment of the present invention. It will, ofcourse, be appreciated that the digital ink (“interaction data”) sentfrom the pen 101 to the netpage system in the form of digital ink mayinclude other data e.g. a digital signature, pen mode (see US2007/125860), orientation data, pen ID, nib ID etc.

An example of interpreting digital ink, received by the netpage systemfrom the netpage pen 101, is discussed briefly above. A more detaileddiscussion of how the netpage system may interpret interaction data canbe found in the Applicant's previously-filed applications (see, forexample, US 2007/130117 and US 2007/108285, the contents of which areherein incorporated by reference).

3. Netpage Pen 3.1 Functional Overview

The active sensing device of the netpage system may take the form of aclicker (for clicking on a specific position on a surface), a pointerhaving a stylus (for pointing or gesturing on a surface using pointerstrokes), or a pen having a marking nib (for marking a surface with inkwhen pointing, gesturing or writing on the surface). For a descriptionof various netpage sensing devices, reference is made to U.S. Pat. No.7,105,753; U.S. Pat. No. 7,015,901; U.S. Pat. No. 7,091,960; and USPublication No. 2006/0028459, the contents of each of which are hereinincorporated by reference.

It will be appreciated that the present invention may utilize anysuitable optical reader. However, the Netpage pen 400 will be describedherein as one such example.

The Netpage pen 400 is a motion-sensing writing instrument which worksin conjunction with a tagged Netpage surface (see Section 2). The penincorporates a conventional ballpoint pen cartridge for marking thesurface, an image sensor and processor for simultaneously capturing theabsolute path of the pen on the surface and identifying the surface, aforce sensor for simultaneously measuring the force exerted on the nib,and a real-time clock for simultaneously measuring the passage of time.

While in contact with a tagged surface, as indicated by the forcesensor, the pen continuously images the surface region adjacent to thenib, and decodes the nearest tag in its field of view to determine boththe identity of the surface, its own instantaneous position on thesurface and the pose of the pen. The pen thus generates a stream oftimestamped position samples relative to a particular surface, andtransmits this stream to the Netpage server 10. The sample streamdescribes a series of strokes, and is conventionally referred to asdigital ink (DInk). Each stroke is delimited by a pen down and a pen upevent, as detected by the force sensor. More generally, any dataresulting from an interaction with a Netpage, and transmitted to theNetpage server 10, is referred to herein as “interaction data”.

The pen samples its position at a sufficiently high rate (nominally 100Hz) to allow a Netpage server to accurately reproduce hand-drawnstrokes, recognise handwritten text, and verify hand-written signatures.

The Netpage pen also supports hover mode in interactive applications. Inhover mode the pen is not in contact with the paper and may be somesmall distance above the surface of the paper (or other substrate). Thisallows the position of the pen, including its height and pose to bereported. In the case of an interactive application the hover modebehaviour can be used to move a cursor without marking the paper, or thedistance of the nib from the coded surface could be used for toolbehaviour control, for example an air brush function.

The pen includes a Bluetooth radio transceiver for transmitting digitalink via a relay device to a Netpage server. When operating offline froma Netpage server the pen buffers captured digital ink in non-volatilememory. When operating online to a Netpage server the pen transmitsdigital ink in real time.

The pen is supplied with a docking cradle or “pod”. The pod contains aBluetooth to USB relay. The pod is connected via a USB cable to acomputer which provides communications support for local applicationsand access to Netpage services.

The pen is powered by a rechargeable battery. The battery is notaccessible to or replaceable by the user. Power to charge the pen can betaken from the USB connection or from an external power adapter throughthe pod. The pen also has a power and USB-compatible data socket toallow it to be externally connected and powered while in use.

The pen cap serves the dual purpose of protecting the nib and theimaging optics when the cap is fitted and signalling the pen to leave apower-preserving state when uncapped.

3.2 Ergonomics and Layout

FIG. 18 shows a rounded triangular profile gives the pen 400 anergonomically comfortable shape to grip and use the pen in the correctfunctional orientation. It is also a practical shape for accommodatingthe internal components. A normal pen-like grip naturally conforms to atriangular shape between thumb 402, index finger 404 and middle finger406.

As shown in FIG. 19, a typical user writes with the pen 400 at a nominalpitch of about 30 degrees from the normal toward the hand 408 when held(positive angle) but seldom operates a pen at more than about 10 degreesof negative pitch (away from the hand). The range of pitch angles overwhich the pen 400 is able to image the pattern on the paper has beenoptimised for this asymmetric usage. The shape of the pen 400 helps toorient the pen correctly in the user's hand 408 and to discourage theuser from using the pen “upside-down”. The pen functions “upside-down”but the allowable tilt angle range is reduced.

The cap 410 is designed to fit over the top end of the pen 400, allowingit to be securely stowed while the pen is in use. Multi colour LEDsilluminate a status window 412 in the top edge (as in the apex of therounded triangular cross section) of the pen 400 near its top end. Thestatus window 412 remains un-obscured when the cap is stowed. Avibration motor is also included in the pen as a haptic feedback system(described in detail below).

As shown in FIG. 20, the grip portion of the pen has a hollow chassismolding 416 enclosed by a base molding 528 to house the othercomponents. The ink cartridge 414 for the ball point nib (not shown)fits naturally into the apex 420 of the triangular cross section,placing it consistently with the user's grip. This in turn providesspace for the main PCB 422 in the centre of the pen and for the battery424 in the base of the pen. By referring to FIG. 21A, it can be seenthat this also naturally places the tag-sensing optics 426 unobtrusivelybelow the nib 418 (with respect to nominal pitch). The nib molding 428of the pen 400 is swept back below the ink cartridge 414 to preventcontact between the nib molding 428 and the paper surface when the penis operated at maximum pitch.

As best shown in FIG. 21B, the imaging field of view 430 emerges througha centrally positioned IR filter/window 432 below the nib 418, and twonear-infrared illumination LEDs 434, 436 emerge from the two bottomcorners of the nib molding 428. Each LED 434, 436 has a correspondingillumination field 438, 440.

As the pen is hand-held, it may be held at an angle that causesreflections from one of the LED's that are detrimental to the imagesensor. By providing more than one LED, the LED causing the offendingreflections can be extinguished.

Specific details of the pen mechanical design can be found in USPublication No. 2006/0028459, the contents of which are hereinincorporated by reference.

3.3 Pen Feedback Indications

FIG. 22 is a longitudinal cross section through the centre-line if thepen 400 (with the cap 410 stowed on the end of the pen). The penincorporates red and green LEDs 444 to indicate several states, usingcolours and intensity modulation. A light pipe 448 on the LEDs 444transmit the signal to the status indicator window 412 in the tubemolding 416. These signal status information to the user includingpower-on, battery level, untransmitted digital ink, network connectionon-line, fault or error with an action, detection of an “active area”flag, detection of an “embedded data” flag, further data sampling torequired to acquire embedded data, acquisition of embedded datacompleted etc.

A vibration motor 446 is used to haptically convey information to theuser for important verification functions during transactions. Thissystem is used for important interactive indications that might bemissed due to inattention to the LED indicators 444 or high levels ofambient light. The haptic system indicates to the user when:

-   -   The pen wakes from standby mode    -   There is an error with an action    -   To acknowledge a transaction

3.4 Pen Optics

The pen incorporates a fixed-focus narrowband infrared imaging system.It utilizes a camera with a short exposure time, small aperture, andbright synchronised illumination to capture sharp images unaffected bydefocus blur or motion blur.

TABLE 10 Optical Specifications Magnification ^(~)0.225 Focal length oflens 6.0 mm Viewing distance 30.5 mm Total track length 41.0 mm Aperturediameter 0.8 mm Depth of field .^(~)/6.5 mm Exposure time 200 usWavelength 810 nm Image sensor size 140 × 140 pixels Pixel size 10 umPitch range ^(~)15

 45 deg Roll range ^(~)30

 30 deg Yaw range 0 to 360 deg Minimum sampling rate 2.25 pixels permacrodot Maximum pen velocity 0.5 m/s ¹Allowing 70 micron blur radius²Illumination and filter ³Pitch, roll and yaw are relative to the axisof the pen

Cross sections showing the pen optics are provided in FIGS. 23A and 23B.An image of the Netpage tags printed on a surface 548 adjacent to thenib 418 is focused by a lens 488 onto the active region of an imagesensor 490. A small aperture 494 ensures the available depth of fieldaccommodates the required pitch and roll ranges of the pen 400.

First and second LEDs 434 and 436 brightly illuminate the surface 549within the field of view 430. The spectral emission peak of the LEDs ismatched to the spectral absorption peak of the infrared ink used toprint Netpage tags to maximise contrast in captured images of tags. Thebrightness of the LEDs is matched to the small aperture size and shortexposure time required to minimise defocus and motion blur.

A longpass IR filter 432 suppresses the response of the image sensor 490to any coloured graphics or text spatially coincident with imaged tagsand any ambient illumination below the cut-off wavelength of the filter432. The transmission of the filter 432 is matched to the spectralabsorption peak of the infrared ink to maximise contrast in capturedimages of tags. The filter also acts as a robust physical window,preventing contaminants from entering the optical assembly 470.

3.5 Pen Imaging System

A ray trace of the optic path is shown in FIG. 24. The image sensor 490is a CMOS image sensor with an active region of 140 pixels squared. Eachpixel is 10 μm squared, with a fill factor of 93%. Turning to FIG. 25,the lens 488 is shown in detail. The dimensions are:

-   -   D=3 mm    -   R1=3.593 mm    -   R2=15.0 mm    -   X=0.8246 mm    -   Y=1.0 mm    -   Z=0.25 mm

This gives a focal length of 6.15 mm and transfers the image from theobject plane (tagged surface 548) to the image plane (image sensor 490)with the correct sampling frequency to successfully decode all imagesover the specified pitch, roll and yaw ranges. The lens 488 is biconvex,with the most curved surface facing the image sensor. The minimumimaging field of view 430 required to guarantee acquisition ofsufficient tag data with each interaction is dependent on the specificcoding pattern. The required field of view for the coding pattern of thepresent invention is described in Section 2.10.

The required paraxial magnification of the optical system is defined bythe minimum spatial sampling frequency of 2.25 pixels per macrodot forthe fully specified tilt range of the pen 400, for the image sensor 490of 10 μm pixels. Typically, the imaging system employs a paraxialmagnification of 0.225, the ratio of the diameter of the inverted imageat the image sensor to the diameter of the field of view at the objectplane, on an image sensor 490 of minimum 128×128 pixels. The imagesensor 490 however is 140×140 pixels, in order to accommodatemanufacturing tolerances. This allows up to +/−120 μm (12 pixels in eachdirection in the plane of the image sensor) of misalignment between theoptical axis and the image sensor axis without losing any of theinformation in the field of view.

The lens 488 is made from Poly-methyl-methacrylate (PMMA), typicallyused for injection moulded optical components. PMMA is scratchresistant, and has a refractive index of 1.49, with 90% transmission at810 nm. The lens is biconvex to assist moulding precision and features amounting surface to precisely mate the lens with the optical barrelmolding 492.

A 0.8 mm diameter aperture 494 is used to provide the depth of fieldrequirements of the design.

The specified tilt range of the pen is 15.0 to 45.0 degree pitch, with aroll range of 30.0 to 30.0 degrees. Tilting the pen through itsspecified range moves the tilted object plane up to 6.3 mm away from thefocal plane. The specified aperture thus provides a corresponding depthof field of 76.5 mm, with an acceptable blur radius at the image sensorof 16 μm.

Due to the geometry of the pen design, the pen operates correctly over apitch range of 33.0 to 45.0 degrees.

Referring to FIG. 26, the optical axis 550 is pitched 0.8 degrees awayfrom the nib axis 552. The optical axis and the nib axis converge towardthe paper surface 548. With the nib axis 552 perpendicular to the paper,the distance A between the edge of the field of view 430 closest to thenib axis and the nib axis itself is 1.2 mm.

The longpass IR filter 432 is made of CR-39, a lightweight thermosetplastic heavily resistant to abrasion and chemicals such as acetone.Because of these properties, the filter also serves as a window. Thefilter is 1.5 mm thick, with a refractive index of 1.50. Each filter maybe easily cut from a large sheet using a CO₂ laser cutter.

3.6 Electronics Design

TABLE 11 Electrical Specifications Processor ARM7 (Atmel AT91FR40162)running at 80 MHz with 256 kB SRAM and 2 MB flash memory Digital inkstorage 5 hours of writing capacity Bluetooth Compliance 1.2 USBCompliance 1.1 Battery standby time 12 hours (cap off), >4 weeks (capon) Battery writing time 4 hours of cursive writing (81% pen down,assuming easy offload of digital ink) Battery charging time 2 hoursBattery Life Typically 300 charging cycles or 2 years (whichever occursfirst) to 80% of initial capacity. Battery Capacity/Type ~340 mAh at 3.7V, Lithium-ion Polymer (LiPo)

FIG. 27 is a block diagram of the pen electronics. The electronicsdesign for the pen is based around five main sections. These are:

-   -   the main ARM7 microprocessor 574,    -   the image sensor and image processor 576,    -   the Bluetooth communications module 578,    -   the power management unit IC (PMU) 580 and    -   the force sensor microprocessor 582.

3.6.1 Microprocessor

The pen uses an Atmel AT91FR40162 microprocessor (see Atmel, AT91 ARMThumb Microcontrollers—AT91FR40162 Preliminary,http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytesof on-chip single wait state SRAM and 2 MBytes of external flash memoryin a stack chip package.

This microprocessor 574 forms the core of the pen 400. Its dutiesinclude:

-   -   setting up the Jupiter image sensor 584,    -   decoding images of Netpage coding pattern (see Section 2.10),        with assistance from the image processing features of the image        sensor 584, for inclusion in the digital ink stream along with        force sensor data received from the force sensor microprocessor        582,    -   setting up the power management IC (PMU) 580,    -   compressing and sending digital ink via the Bluetooth        communications module 578, and    -   programming the force sensor microprocessor 582.

The ARM7 microprocessor 574 runs from an 80 MHz oscillator. Itcommunicates with the Jupiter image sensor 576 using a UniversalSynchronous Receiver Transmitter (USRT) 586 with a 40 MHz clock. TheARM7 574 communicates with the Bluetooth module 578 using a UniversalAsynchronous Receiver Transmitter (UART) 588 running at 115.2 kbaud.Communications to the PMU 580 and the Force Sensor microProcessor (FSP)582 are performed using a Low Speed Serial bus (LSS) 590. The LSS isimplemented in software and uses two of the microprocessor's generalpurpose IOs.

The ARM7 microprocessor 574 is programmed via its JTAG port.

3.6.2 Image Sensor

The ‘Jupiter’ Image Sensor 584 (see US Publication No. 2005/0024510, thecontents of which are incorporated herein by reference) contains amonochrome sensor array, an analogue to digital converter (ADC), a framestore buffer, a simple image processor and a phase lock loop (PLL). Inthe pen, Jupiter uses the USRT's clock line and its internal PLL togenerate all its clocking requirements. Images captured by the sensorarray are stored in the frame store buffer. These images are decoded bythe ARM7 microprocessor 574 with help from the ‘Callisto’ imageprocessor contained in Jupiter. The Callisto image processor performs,inter alia, low-pass filtering of captured images (see Section 2.10 andUS Publication No. 2005/0024510) before macrodot sampling and decodingby the microprocessor 574.

Jupiter controls the strobing of two infrared LEDs 434 and 436 at thesame time as its image array is exposed. One or other of these twoinfrared LEDs may be turned off while the image array is exposed toprevent specular reflection off the paper that can occur at certainangles.

3.6.3 Bluetooth Communications Module

The pen uses a CSR BlueCore4-External device (see CSR,BlueCore4-External Data Sheet rev c, 6 Sep. 2004) as the Bluetoothcontroller 578. It requires an external 8 Mbit flash memory device 594to hold its program code. The BlueCore4 meets the Bluetooth v1.2specification and is compliant to v0.9 of the Enhanced Data Rate (EDR)specification which allows communication at up to 3 Mbps.

A 2.45 GHz chip antenna 486 is used on the pen for the Bluetoothcommunications.

The BlueCore4 is capable of forming a UART to USB bridge. This is usedto allow USB communications via data/power socket 458 at the top of thepen 456.

Alternatives to Bluetooth include wireless LAN and PAN standards such asIEEE 802.11 (Wi-Fi) (see IEEE, 802.11 Wireless Local Area Networks,http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (seeIEEE, 802.15 Working Group for WPAN,http://grouper.ieee.org/groups/802/15/index.html), ZigBee (see ZigBeeAlliance, http://www.zigbee.org), and WirelessUSB Cypress (seeWirelessUSB LR 2.4-GHz DSSS Radio SoC,http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as wellas mobile standards such as GSM (see GSM Association,http://www.gsmworld.com/index.shtml), GPRS/EDGE, GPRS Platform,http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMADevelopment Group, http://www.cdg.org/, and Qualcomm,http://www.qualcomm.com), and UMTS (see 3rd Generation PartnershipProject (3GPP), http://www.3gpp.org).

3.6.4 Power Management Chip

The pen uses an Austria Microsystems AS3603 PMU 580 (see AustriaMicrosystems, AS3603 Multi-Standard Power Management Unit Data Sheetv2.0). The PMU is used for battery management, voltage generation, powerup reset generation and driving indicator LEDs and the vibrator motor.

The PMU 580 communicates with the ARM7 microprocessor 574 via the LSSbus 590.

3.6.5 Force Sensor Subsystem

The force sensor subsystem comprises a custom Hokuriku force sensor 500(based on Hokuriku, HFD-500 Force Sensor,http://www.hdk.co.jp/pdf/eng/e1381AA.pdf), an amplifier and low passfilter 600 implemented using op-amps and a force sensor microprocessor582.

The pen uses a Silicon Laboratories C8051F330 as the force sensormicroprocessor 582 (see Silicon Laboratories, C8051F330/1 MCU DataSheet, rev 1.1). The C8051F330 is an 8051 microprocessor with on chipflash memory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5MHz oscillator and also uses an external 32.768 kHz tuning fork.

The Hokuriku force sensor 500 is a silicon piezoresistive bridge sensor.An op-amp stage 600 amplifies and low pass (anti-alias) filters theforce sensor output. This signal is then sampled by the force sensormicroprocessor 582 at 5 kHz.

Alternatives to piezoresistive force sensing include capacitive andinductive force sensing (see Wacom, “Variable capacity condenser andpointer”, US Patent Application 20010038384, filed 8 Nov. 2001, andWacom, Technology, http://www.wacom-components.com/english/tech.asp).

The force sensor microprocessor 582 performs further (digital) filteringof the force signal and produces the force sensor values for the digitalink stream. A frame sync signal from the Jupiter image sensor 576 isused to trigger the generation of each force sample for the digital inkstream. The temperature is measured via the force sensormicroprocessor's 582 on chip temperature sensor and this is used tocompensate for the temperature dependence of the force sensor andamplifier. The offset of the force signal is dynamically controlled byinput of the microprocessor's DAC output into the amplifier stage 600.

The force sensor microprocessor 582 communicates with the ARM7microprocessor 574 via the LSS bus 590. There are two separate interruptlines from the force sensor microprocessor 582 to the ARM7microprocessor 574. One is used to indicate that a force sensor sampleis ready for reading and the other to indicate that a pen down/up eventhas occurred.

The force sensor microprocessor flash memory is programmed in-circuit bythe ARM7 microprocessor 574.

The force sensor microprocessor 582 also provides the real time clockfunctionality for the pen 400. The RTC function is performed in one ofthe microprocessor's counter timers and runs from the external 32.768kHz tuning fork. As a result, the force sensor microprocessor needs toremain on when the cap 472 is on and the ARM7 574 is powered down. Hencethe force sensor microprocessor 582 uses a low power LDO separate fromthe PMU 580 as its power source. The real time clock functionalityincludes an interrupt which can be programmed to power up the ARM7 574.

The cap switch 602 is monitored by the force sensor microprocessor 582.When the cap assembly 472 is taken off (or there is a real time clockinterrupt), the force sensor microprocessor 582 starts up the ARM7 572by initiating a power on and reset cycle in the PMU 580.

3.7 Pen Software

The Netpage pen software comprises that software running onmicroprocessors in the Netpage pen 400 and Netpage pod.

The pen contains a number of microprocessors, as detailed in Section3.6. The Netpage pen software includes software running on the AtmelARM7 CPU 574 (hereafter CPU), the Force Sensor microprocessor 582, andalso software running in the VM on the CSR BlueCore Bluetooth module 578(hereafter pen BlueCore). Each of these processors has an associatedflash memory which stores the processor specific software, together withsettings and other persistent data. The pen BlueCore 578 also runsfirmware supplied by the module manufacturer, and this firmware is notconsidered a part of the Netpage pen software.

The pod contains a CSR BlueCore Bluetooth module (hereafter podBlueCore). The Netpage pen software also includes software running inthe VM on the pod BlueCore.

As the Netpage pen 400 traverses a Netpage tagged surface 548, a streamof correlated position and force samples are produced. This stream isreferred to as DInk Note that DInk may include samples with zero force(so called “Hover DInk”) produced when the Netpage pen is in proximityto, but not marking, a Netpage tagged surface.

The CPU component of the Netpage pen software is responsible for DInkcapture, tag image processing and decoding (in conjunction with theJupiter image sensor 576), storage and offload management, hostcommunications, user feedback and software upgrade. It includes anoperating system (RTOS) and relevant hardware drivers. In addition, itprovides a manufacturing and maintenance mode for calibration,configuration or detailed (non-field) fault diagnosis. The Force Sensormicroprocessor 582 component of the Netpage pen software is responsiblefor filtering and preparing force samples for the main CPU. The penBlueCore VM software is responsible for bridging the CPU UART 588interface to USB when the pen is operating in tethered mode. The penBlueCore VM software is not used when the pen is operating in Bluetoothmode.

The pod BlueCore VM software is responsible for sensing when the pod ischarging a pen 400, controlling the pod LEDs appropriately, andcommunicating with the host PC via USB.

For a detailed description of the software modules, reference is made toUS Publication No. 2006/0028459, the contents of which are hereinincorporated by reference.

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

1. A substrate having a coding pattern disposed thereon or therein, saidcoding pattern comprising: a tiling of contiguous grid cells, each gridcell being demarcated by t target elements and having t-fold rotationalsymmetry, each grid cell containing nt registration symbols, eachregistration symbol being encoded by a set of macrodots; and a tiling ofcontiguous tags, each tag consisting of an array of c grid cells, eachtag containing a plurality of data symbols and having an identicallayout of data symbols, each data symbol being encoded by a set ofmacrodots; wherein: the coding pattern has a physical layout defined byits tiling of contiguous grid cells, said physical layout belonging to aplane symmetry group that has t-fold rotational symmetry andtranslational symmetry with the grid cell as its unit cell; the codingpattern has a logical layout defined by its tiling of contiguous tags,said logical layout belonging to a plane symmetry group that has norotational symmetry but has translational symmetry with the tag as itsunit cell; there are ct possible registrations between the physicallayout of the coding pattern and the logical layout of the codingpattern, each registration corresponding to a distinct combination ofone of the t possible rotations of the physical layout of coding patternrelative to the logical layout of the coding pattern and one of the cpossible translations of the physical layout of the coding patternrelative to the logical layout of the coding pattern; any contiguoustag-shaped array of c grid cells contains cnt registration symbols, saidregistration symbols, taken in a defined sequence relative to thephysical layout of the tag-shaped array, forming a registration codewordof length r; there are v distinct registration codewords, eachcorresponding to a distinct one of the ct possible registrations; theregistration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between said tag-shapedarray and the logical layout of the coding pattern; t=is an integervalue of 2 or more; c=is an integer value of 2 or more; n is an integervalue of 1 or more;cnt≧r; andv≧ct.
 2. The substrate of claim 1, wherein n=1 and r=ct.
 3. Thesubstrate of claim 1, wherein: t is 2, 3, 4 or 6; c is 2, 3, 4, 9, 16,25 or 36; and n is 1, 2, 3 or
 4. 4. The substrate of claim 1, whereinthe registration codeword of each tag-shaped array facilitatesidentification of at least one of the data symbols of a tag that sharesat least one grid cell with said tag-shaped array.
 5. The substrate ofclaim 1, wherein v=ct.
 6. The substrate of claim 5, wherein a minimumdistance between each of the ct distinct registration codewords is atleast r/2, thereby allowing the registration between said tag-shapedarray and the logical layout of the coding pattern to be determined inthe presence of up to (r/2−1)/2 registration symbol errors.
 7. Thesubstrate of claim 1, wherein v>ct, such that each of the ct possibleregistrations has a plurality of corresponding registration codewords.8. The substrate of claim 7, wherein a minimum distance between pairs ofdistinct registration codewords encoding different registrations is atleast r/2.
 9. The substrate of claim 8, wherein each of the ct possibleregistrations has first and second corresponding registration codewords,said first registration codewords identifying a first encoding format ofsaid coding pattern and said second registration codewords identifying asecond encoding format of said coding pattern.
 10. The substrate ofclaim 7, wherein a minimum distance (d_(format)) between each pair offirst and second registration codewords encoding the same registrationis at least r/2.
 11. The substrate of claim 9, wherein each data symbolis represented by d₁ macrodots, each of said d₁ macrodots occupying arespective position from a plurality of predetermined possible positionsp₁, the respective positions of said d₁ macrodots representing one of aplurality of possible data values, and wherein d₁ has different valuesin said first and second encoding formats.
 12. The substrate of claim 9,wherein each of the ct possible registrations has a correspondingflagged first registration codeword, a corresponding unflagged firstregistration codeword, a corresponding flagged second registrationcodeword and a corresponding unflagged second registration codeword,said flagged registration codewords identifying the presence of anactive area flag and said unflagged registration codewords identifyingthe absence of said active area flag.
 13. The substrate of claim 11,wherein: at least some tag-shaped arrays in said coding pattern havingthe first encoding format contain a mixture of registration symbols fromsaid flagged first registration codeword and registration symbols fromsaid unflagged first registration codeword to form a mixed firstregistration codeword of length r; and at least some tag-shaped arraysin said coding pattern having the second format contain a mixture ofregistration symbols from said flagged second registration codeword andregistration symbols from said unflagged second registration codeword toform a mixed second registration codeword of length r.
 14. The substrateof claim 13, wherein a minimum distance (d_(flag)) between each pair offlagged and unflagged registration codewords encoding the sameregistration and the same format is less than r/2, thereby enablingcorrection of registration symbol errors in said mixed first and secondregistration codewords.
 15. The substrate of claim 14, wherein each tagcontains a registration codeword selected from one of the codewordsdefined as: codeword active area codeword type tag format flag 4414,0332, 2253, 1132, 1411, first; unflagged 0 0 3402, 5503, 2113, 21552414, 3332, 4253, 4132, 4411, first; flagged 1 5402, 4503, 1113, 31552321, 5555, 1101, 1212, 5500, second; 1 0 4540, 5531, 4311, 4512unflagged 0321, 3555, 0101, 0212, 4500, second; 1 5540, 1531, 5311, 5512flagged


16. The substrate of claim 1, wherein each registration symbol isrepresented by d₂ macrodots, each of said d₂ macrodots occupying arespective position from a plurality of predetermined possible positionsp₂, the respective positions of said d₂ macrodots representing one of aplurality of possible registration symbol values.
 17. The substrate ofclaim 1, wherein each tag comprises at least one local codewordidentifying a location of a respective tag, said local codewordcomprising a respective set of said data symbols.
 18. The substrate ofclaim 17, wherein each tag comprises one or more common codewords, eachcommon codeword being common to a plurality of contiguous tags, eachcommon codeword comprising a respective set of said data symbols. 19.The substrate of claim 18, wherein each common codeword, or a set ofcommon codewords, identifies a region identity, a page identity or asubstrate identity.
 20. A substrate having a coding pattern disposed onor in a surface thereof, said coding pattern comprising: a tiling ofcontiguous grid cells, each grid cell being demarcated by t targetelements and having t-fold rotational symmetry, each grid cellcontaining nt registration symbols, each registration symbol beingencoded by a set of macrodots; and a tiling of contiguous tags, each tagconsisting of an array of c grid cells, each tag containing a pluralityof data symbols and having an identical layout of data symbols, eachdata symbol being encoded by a set of macrodots; wherein: the codingpattern has a physical layout defined by its tiling of contiguous gridcells, said physical layout belonging to a plane symmetry group that hasat least one reflection axis, t-fold rotational symmetry andtranslational symmetry with the grid cell as its unit cell; the codingpattern has a logical layout defined by its tiling of contiguous tags,said logical layout belonging to a plane symmetry group that has noreflection axis and no rotational symmetry, but has translationalsymmetry with the tag as its unit cell; there are 2ct possibleregistrations between the physical layout of the coding pattern and thelogical layout of the coding pattern, each registration corresponding toa distinct combination of: (1) whether or not the physical layout of thecoding pattern is reflected relative to the logical layout of the codingpattern; (2) one of t possible rotations of the physical layout ofcoding pattern relative to the logical layout of the coding pattern; and(3) one of c possible translations of the physical layout of the codingpattern relative to the logical layout of the coding pattern; anycontiguous tag-shaped array of c grid cells contains cnt registrationsymbols, said registration symbols, taken in a defined sequence relativeto the physical layout of the tag-shaped array, forming a registrationcodeword of length r; there are v distinct registration codewords, eachcorresponding to a distinct one of the 2ct possible registrations; theregistration codeword of each contiguous tag-shaped array of c gridcells thereby uniquely identifies a registration between said tag-shapedarray and the logical layout of the coding pattern; t=is an integervalue of 2 or more; c=is an integer value of 2 or more; n is an integervalue of 1 or more;cnt≧r; andv≧2ct.